Anali za istrske in mediteranske študije
Annali di Studi istriani e mediterranei
Annals for Istrian and Mediterranean Studies
Series Historia Naturalis, 35, 2025, 1
UDK 5                       Annales, Ser. hist. nat., 35, 2025, 1, pp. 1-170, Koper 2025  ISSN 1408-533X
KOPER 2025
Anali za istrske in mediteranske študije
Annali di Studi istriani e mediterranei
Annals for Istrian and Mediterranean Studies
Series Historia Naturalis, 35, 2025, 1
UDK 5                    ISSN 1408-533X 
                e-ISSN 2591-1783
ANNALES · Ser. hist. nat. · 35 · 2025 · 1
Anali za istrske in mediteranske študije - Annali di Studi istriani e mediterranei - Annals for Istrian and Mediterranean Studies
ISSN 1408-533X     UDK 5  Letnik 35, leto 2025 številka 1
e-ISSN 2591-1783
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ANNALES · Ser. hist. nat. · 35 · 2025 · 1
Anali za istrske in mediteranske študije - Annali di Studi istriani e mediterranei - Annals for Istrian and Mediterranean Studies
UDK 5                                                      Letnik 35, Koper 2025, številka 1                                      ISSN 1408-53 3X
e-ISSN 2591-1783 
VSEBINA / INDICE GENERALE / CONTENTS
BIOTSKA GLOBALIZACIJA
GLOBALIZZAZIONE BIOTICA
BIOTIC GLOBALIZATION
Okan AKYOL, Oğuzhan TAKICAK & 
Hasan TARUN
A Fugitive Lessepsian Fish in a Sea-Cage 
Farm in the Aegean Sea: Stephanolepis 
diaspros (Monacanthidae) ................................
Ubežni lesepski migrant iz ribogojnice 
v Egejskem morju: Stephanolepis 
diaspros (Monacanthidae)
Nicola BETTOSO, Lisa FARESI, Valentina 
TORBOLI & Jose A. CUESTA
Additional Record of the Pea Crab 
Pinnotheres bicristatus (Brachyura: 
Pinnotheridae) in the Adriatic Sea ....................
Dodatni zapis o pojavljanju stražne 
rakovice vrste Pinnotheres bicristatus 
(Brachyura: Pinnotheridae) 
v Jadranskem morju
Alan DEIDUN, Sarah BAUMANN, 
Bruno ZAVA & Maria CORSINI-FOKA
Confirming the Occurrence of the 
Non-Indigenous Pteragogus trispilus 
(Actinopterygii: Labridae) within 
Maltese Waters ................................................
Potrditev pojavljanja tujerodne ustnače 
vrste Pteragogus trispilus (Actinopterygii: 
Labridae) znotraj malteških voda
Deniz ERGÜDEN, Yusuf Kenan BAYHAN, 
Sibel ALAGÖZ ERGÜDEN & Deniz AYAS
A New Ichthyological Record and 
Distributional Update for Epigonus 
denticulatus Dieuzeide, 1950 in 
Turkish Mediterranean Waters ...........................
Nov ihtiološki zapis in podatki o 
razširjenosti rjavega veleokca, Epigonus 
denticulatus Dieuzeide, 1950 v turških 
sredozemskih vodah
Sara LADOUL, Farid HEMIDA, 
Christian REYNAUD & Christian CAPAPÉ
On the Occurrence of Cornish Blackfish 
Schedophilus medusophagus 
(Osteichthyes: Centrolophidae) 
from the Maghreb Shore 
(Southwestern Mediterranean Sea) ...................
Potrjena prisotnost meduzojeda 
Schedophilus medusophagus 
(Osteichthyes: Centrolophidae) 
z magrebske obale (jugozahodno 
Sredozemsko morje)
Christina MICHAIL & Francesco TIRALONGO
First Occurrence of Ariidae in 
Cypriot Waters – a Major 
Contribution to Biodiversity .............................
Prvo pojavljanje predstavnikov iz 
družine Ariidae v ciprskih vodah – 
velik prispevek k biodiverziteti
SREDOZEMSKE HRUSTANČNICE
SQUALI E RAZZE MEDITERRANEE
MEDITERRANEAN SHARKS AND RAYS
Lovrenc LIPEJ, Riccardo BATTISTELLA, 
Borut MAVRIČ & Danijel IVAJNŠIČ
An Insight into the Diet of the Bull Ray, 
Aetomylaeus bovinus (Geoffroy 
Saint-Hilaire, 1817) in the 
Northern Adriatic Sea ......................................
Vpogled v prehranjevalne navade 
kljunatega morskega goloba, 
Aetomylaeus bovinus (Geoffroy 
Saint-Hilaire, 1817) v severnem Jadranu
Cem ÇEVİK, Deniz ERGÜDEN & Deniz AYAS
A New Capture Record of Alopias 
superciliosus Lowe, 1841 from the 
Turkish Coast (Northeastern Mediterranean) ......
Nov ulov velikooke morske lisice 
Alopias superciliosus Lowe, 1841 
iz turške obale (severovzhodno Sredozemlje)
1
7
13
21
35
27
43
55
ANNALES · Ser. hist. nat. · 35 · 2025 · 1
Cem DALYAN, N. Bikem KESİCİ, Elif YÜCEDAĞ 
BAKIR, Yunus GÖNÜL & Hakan KABASAKAL
No Longer as Common as its Name: a Review 
of the Occurrence of Torpedo torpedo 
(Linnaeus, 1758) (Chondrichthyes: 
Elasmobranchii) in Turkish Waters, 
with Photographic Evidence .............................
Ni več tako pogost kot njegovo ime: pregled 
pojavljanja okatega električnega skata Torpedo 
torpedo (Linnaeus, 1758) (Chondrichthyes: 
Elasmobranchii) v turških vodah s 
fotografskimi dokazi
Deniz ERGÜDEN, Cemal TURAN, Servet Ahmet 
DOĞDU & Deniz AYAS 
Disc Deformity in a Juvenile Female Brown Ray, 
Raja miraletus (Family: Rajidae), from 
Northeastern Mediterranean (Türkiye) .................
Deformacija diska pri juvenilni samici modropege 
raže, Raja miraletus (družina: Rajidae), iz 
severovzhodnega Sredozemskega morja (Turčija)
Farid HEMIDA, Christian REYNAUD & 
Christian CAPAPÉ 
On an Old Record of the Smalltooth Sand Tiger 
Shark Odontaspis ferox (Chondrichthyes: 
Odontaspididae) from the Algerian Coast 
(Southwestern Mediterranean Sea) ........................
O starem zapisu o drobnozobem morskem biku 
Odontaspis ferox (Chondrichthyes: Odontaspididae) 
z alžirske obale (jugozahodno Sredozemsko morje)
Hakan KABASAKAL, Uğur UZER & 
F. Saadet KARAKULAK
Plastic Debris-Induced Fin Damage 
in the Smoothhound, Mustelus mustelus ..............
Poškodbe plavuti pri navadnem 
morskem psu, Mustelus mustelus, 
zaradi plastičnih odpadkov
Nicolas ZIANI, Florane TONDU, 
Rémi BRU, Chloé MOSNIER, 
Sarah FOXONET, Ruben Bao GALLIEN, 
Mathias POULY, Modan Lou TONIETTO, 
Lucille VERDON, Eloïse DEYSSON, 
Alessandro DE MADDALENA & 
Hakan KABASAKAL 
Bite Marks Observed on a Large Female 
White Shark Carcharodon carcharias 
Off Camargue, France Provide Potential 
Insights into the Reproduction of the 
Mediterranean Population ................................
Sledovi ugrizov na veliki samici 
belega morskega volka Carcharodon 
carcharias pri Camargu (Francija) 
kažejo na možno razmnoževanje 
sredozemske populacije
MORSKA FAVNA
FAUNA MARINA
MARINE FAUNA
Sihem RAFRAFI-NOUIRA, RIMEL 
BENMESSAOUD, Mourad CHÉRIF, 
Christian REYNAUD & Christian CAPAPÉ
Morphological Deformities in a 
Common Two-Banded Sea Bream, 
Diplodus vulgaris (Osteichthyes: Sparidae), 
from Northern Tunisian Waters 
(Central Mediterranean Sea) ............................
Morfološke deformacije pri fratru, 
Diplodus vulgaris (Osteichthyes: Sparidae), 
iz severnih tunizijskih vod 
(osrednje Sredozemsko morje)
Abdelkarim DERBALI, Aymen HADJ TAIEB & 
Wassim KAMMOUN
The Current Status of Polititapes aureus 
(Mollusca: Bivalvia) in the Coastal 
Zone of Sfax, Tunisia 
(Central Mediterranean) ....................................
Trenutno stanje vrste Polititapes aureus 
(Mollusca: Bivalvia) na obalnem 
območju Sfaxa v Tuniziji 
(osrednje Sredozemlje)
Neža LEBAN & Valentina PITACCO
Current Knowledge on the Distribution 
of the Poorly Known Echiurid Species 
Maxmuelleria gigas (M. Müller, 1852) 
in the Slovenian Sea .........................................
Trenutno poznavanje prostorske 
razporeditve manj poznane vrste 
zvezdaša Maxmuelleria gigas 
(M. Müller, 1852) v slovenskem morju
Jan MALEJ, Tjaša KOGOVŠEK, 
Martin VODOPIVEC, Janja FRANCÉ, 
Patricija MOZETIČ, Matevž MALEJ & 
Alenka MALEJ
Long-Term Study of Zooplankton 
Biomass in the Gulf of Trieste (Adriatic Sea) ......
Dolgoročna študija zooplanktonske 
biomase v Tržaškem zalivu 
(Jadransko morje)
Sihem RAFRAFI-NOUIRA, 
Rimel BENMESSAOUD, Mourad CHÉRIF, 
Christian REYNAUD & Christian CAPAPÉ
Occurrence of the Longjaw Snake Eel, 
Ophisurus serpens (Ophichthidae), in 
Tunisian Waters (Central Mediterranean Sea) .....
Pojavljanje zobate jegulje, Ophisurus 
serpens (Ophichthidae), iz tunizijskih 
voda (osrednje Sredozemsko morje)
83
73
91
65
125
117
109
133
145
97
ANNALES · Ser. hist. nat. · 35 · 2025 · 1
FLORA
FLORA
FLORA
Martina ORLANDO-BONACA, Artur BONACA, 
Diego BONACA & Ana ROTTER
Seagrasses: a Promising Source of 
Bioactive Compounds for Human 
Health Applications ..........................................
Morske cvetnice: obetaven vir 
bioaktivnih spojin za uporabo 
v zdravstvu
OCENE IN POROČILA
RECENSIONI E RELAZIONI
REVIEWS AND REPORTS
Shin-ichi Uye
Book review: Mirrors of the Sea: When 
Science and Art Meet. 30 Years of the 
Unesco Intergovernmental Oceanographic 
Commission in Slovenia ...................................
Kazalo k slikam na ovitku ...................................
Index to images on the cover ............................
153 167
169
169
ANNALES · Ser. hist. nat. · 35 · 2025 · 1
133
received: 2025-03-30                 DOI 10.19233/ASHN.2025.17
LONG-TERM STUDY OF ZOOPLANKTON BIOMASS IN THE SOUTH-
EASTERN PART OF THE GULF OF TRIESTE (ADRIATIC SEA)
Jan MALEJ
Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia
e-mail: jan@malej.si
Tjaša KOGOVŠEK
Center for Marine Research, Rudjer Bošković Institute, G. Paliaga 5, HR-52210 Rovinj, Croatia
Martin VODOPIVEC, Janja FRANCÉ & Patricija MOZETIČ
Marine Biology Station Piran, National Institute of Biology, Fornače 41, SI-6330 Piran, Slovenia
Matevž MALEJ
Faculty of Management, University of Primorska, Izolska vrata 2, SI- 6000 Koper, Slovenia
Alenka MALEJ
Marine Biology Station Piran, National Institute of Biology, Fornače 41, SI-6330 Piran, Slovenia
e-mail: alenka.malej@nib.si
ABSTRACT
We present a dataset of zooplankton biomass spanning more than four decades (1974–2019). The dry 
mass of zooplankton ranged from below 10 to almost 100 mg m-3, was highest in 1989–2002 and declined 
thereafter. The interannual variability of zooplankton dry mass in the period 1993–2018 was compared with 
the regional atmospheric index WeMOi, chlorophyll a and frequency of jellyfish blooms. Increased dry mass 
values were associated with a positive phase of the regional atmospheric index WeMOi, higher chlorophyll a 
and a lower frequency of jellyfish blooms.
Key words: net zooplankton, dry mass, time series, chlorophyll a, jellyfish blooms
STUDIO A LUNGO TERMINE DELLA BIOMASSA ZOOPLANCTONICA NELLA PARTE 
SUD-ORIENTALE DEL GOLFO DI TRIESTE (MARE ADRIATICO)
SINTESI
Gli autori presentano un set di dati sulla biomassa zooplanctonica che copre un arco temporale di oltre 
quattro decenni (1974–2019). Il peso secco dello zooplancton variava da meno di 10 a quasi 100 mg 
m-3, raggiungendo i valori più elevati nel periodo 1989–2002 e diminuendo successivamente. La variabilità 
interannuale del peso secco dello zooplancton nel periodo 1993–2018 è stata confrontata con l’indice 
atmosferico regionale WeMOi, la concentrazione di clorofilla a e la frequenza delle fioriture di meduse. I 
valori più elevati del peso secco sono risultati associati a una fase positiva dell’indice atmosferico regionale 
WeMOi, a una maggiore concentrazione di clorofilla a e a una minore frequenza di fioriture di meduse.
Parole chiave: zooplancton, peso secco, serie storiche, clorofilla a, fioriture di meduse
ANNALES · Ser. hist. nat. · 35 · 2025 · 1
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Jan MALEJ et al.: LONG-TERM STUDY OF ZOOPLANKTON BIOMASS IN THE SOUTH-EASTERN PART OF THE GULF OF TRIESTE (ADRIATIC SEA), 133–144
INTRODUCTION
Zooplankton represents an important trophic step 
in the pelagic food web and connects the primary 
producers with the higher trophic levels. It acts as a 
recycler, converting particulate organic matter into dis-
solved pools, and plays an important role in the bio-
logical pump and carbon export (Steinbeck & Laundry, 
2017). Monitoring zooplankton biomass over extended 
periods of time provides valuable insights into ecosys-
tem health, productivity and resilience. By facilitating 
the assessment of anthropogenic impacts, such data 
can support the development of evidence-based 
management strategies, such as the European Marine 
Strategy Framework Directive (MSFD, 2008/56/EC) and 
the Integrated Monitoring and Assessment Programme 
(IMAP, UNEP/MAP, 2016) of the Barcelona Conven-
tion. Quantitative estimates of zooplankton biomass 
are therefore of paramount importance (Ratnarajah et 
al., 2023), but in general, observational data are sparse.
The most commonly used metrics to determine net 
zooplankton biomass are settling and displacement 
volume (volumetric methods) and wet, dry and ash-free 
dry mass (gravimetric methods). The methodological 
bias is reduced if the displacement volume or wet 
mass is quantified instead of the settling volume, and 
even more so if the dry mass is measured and the fluids 
are eliminated (Postel et al., 2000). While volumetric 
methods were already used in the 19th century to deter-
mine the quantity of net plankton, the measurement of 
dry mass became established as a widely used method 
in the 1960s (Hagen, 2000).
Alternatively, researchers can determine the abun-
dance and taxonomic composition of the plankton 
community and then convert this data into dry mass us-
ing published conversion factors. This method is much 
more time-consuming, and the effort increases if the 
dry mass of a particular taxonomic group or species is 
determined directly, but the results provide additional 
important information. 
The measurement of the bulk dry mass is straight-
forward and standardised in marine ecology studies 
despite some disadvantages. The main drawbacks 
include the destruction of organisms that cannot be 
used for further taxonomic analyses, the inability to 
capture variability due to differences in community 
structure, especially when the abundance of gelatinous 
organisms is considerable, and the lack of information 
on the specific biochemical composition, which is im-
portant for assessing the role of zooplankton in trophic 
dynamics. Nevertheless, the use of standard dry mass 
methods enables consistent results and can therefore 
be used for comparisons across different geographical 
locations and over time.
The main objective of the present study was to 
describe the long-term and interannual fluctuations of 
the zooplankton stock in the northernmost part of the 
Adriatic Sea and to identify possible changes. We re-
port here the temporal patterns of net zooplankton dry 
mass in the Gulf of Trieste over more than four decades 
(1974–2019). We also investigated the relationships 
between zooplankton and phytoplankton biomass as 
well as the regional climate index as possible explana-
tory variables for the observed fluctuations. Finally, 
we use data on the occurrence of jellyfish blooms to 
interpret the observed changes in zooplankton biomass 
and identify a possible top-down control.
MATERIAL AND METHODS
Sample collection and processing
The zooplankton samples were collected in the 
south-eastern part of the Gulf of Trieste (northern 
Adriatic Sea) at the long-term ecological research site 
LTER Gulf of Trieste, Slovenia (LTER GoT SI, 22 m water 
depth, Fig. 1). The Gulf of Trieste is the northernmost 
extension of the Adriatic Sea and is characterised by a 
shallow water depth of less than 20 m on average. Due 
Fig. 1: The site of the long-term ecological research 
station LTER Gulf of Trieste, Slovenia.
Sl. 1: Lokacija dolgoročne ekološke raziskovalne postaje 
LTER, Tržaški zaliv, Slovenija.
ANNALES · Ser. hist. nat. · 35 · 2025 · 1
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Jan MALEJ et al.: LONG-TERM STUDY OF ZOOPLANKTON BIOMASS IN THE SOUTH-EASTERN PART OF THE GULF OF TRIESTE (ADRIATIC SEA), 133–144
to its location and shallow depth, the climatic condi-
tions have a strong influence on the oceanographic and 
biological characteristics. The water column is ther-
mally stratified in the warmer part of the year and well 
mixed in winter. The main freshwater source (the Soča 
River) flows into the Gulf from the north-west coast 
and plays an important role in the bottom-up control 
of phytoplankton (Vascotto et al., 2024). Our sampling 
station is located in the south-eastern part of the Gulf, 
which is more strongly influenced by the EAC (Eastern 
Adriatic Current) and the open waters of the northern 
Adriatic and is considered mesotrophic to oligotrophic 
(Brush et al., 2021).
The time series of zooplankton monitoring with dif-
ferent sampling frequencies was established in 1974. 
However, the monitoring was interrupted several times 
(1976–1978; 1981–1988; 2003) and was carried out 
regularly from 2004 until it was discontinued in 2019. 
The zooplankton samples were collected by vertical 
tows from the near bottom to the surface using three 
different nets: with a mouth opening of 0.25 m2 and 
a mesh size of 212 µm (1974–1975), with an opening 
of 0.5 m2 and a mesh size of 250 µm (1979–1980) and 
since 1989 with the WP2 net, mouth opening of 0.25 
m2 and a mesh size of 200 µm. The volume of water 
filtered was calculated from tow length and net open-
ing. The samples were fixed on board with buffered 
formaldehyde (2%–4% final concentration).
The frequency of sampling varied from five to six 
times a year (1974–1975, 1979–1980, 1989–1992) 
to monthly from 1993 and bi-monthly from 2004. 
Since the beginning of 1993, the seawater samples for 
chlorophyll a determination have been collected using 
5-litre Niskin bottles.
In the laboratory, the zooplankton samples were 
divided into two halves using a Folsom splitter and one 
half-split was used to determine the biomass (dry mass 
– DM). The DM samples were briefly rinsed with fresh-
water, placed in pre-weighed ceramic crucibles, dried 
at 600 C for 24 hours (Lovegrove 1966) and weighed 
to the nearest 0.1 mg. The results were expressed as 
dry mass in milligrams per cubic meter (mg DM m-3). 
The concentrations of chlorophyll a (Chl a), corrected 
for phaeopigments, were determined fluorometrically 
in 90% acetone extracts (Holm-Hansen et al., 1965).
Time series analyses
Monthly time series of the entire DM dataset were 
generated and descriptive statistics (mean, geometric 
mean, standard error, median, standard deviation, 
and 10th and 90th percentiles) were calculated. We 
also calculated the coefficient of variation (CoV) as 
SD/mean. The normalised annual DM anomalies, 
calculated by dividing the anomalies by the long-term 
standard deviation, were estimated for the entire study 
period (1974–2019).
We investigated the relationships of DM with 
phytoplankton biomass (Chl a), the Western Medi-
terranean Oscillation climate index (WeMOi) and 
jellyfish blooms as potential explanatory variables 
for changes in zooplankton biomass. The basic 
working hypothesis was that zooplankton DM was 
influenced by Chl a as proxy of food availability and 
by WeMOi as proxy for environmental conditions. As 
we had several major interruptions in zooplankton 
sampling (1976–1978; 1981–1988) we analysed the 
fluctuations in zooplankton biomass in relation to 
Chl a and the WeMOi only for the last 25 full years 
of the study (1993–2018), using monthly series for 
all parameters. DM results were missing for some 
months (21 out of 301 results), which were scattered 
over the entire period. To fill in these missing values, 
the approx. function in R was used. The same was 
done for missing Chl a values (33 out of 301 results). 
WeMOi measures the difference between the 
standardised  atmospheric pressure in Padua, north-
ern Italy, and  San Fernando, Cádiz, Southwestern 
Spain (Martin-Vide & Lopez-Bustins, 2006) and 
reflects the regional weather conditions. The positive 
phase of WeMO correlates significantly with lower 
sea surface temperatures and higher river discharges 
and has been shown to be associated with increased 
productivity of plankton and small pelagic fish 
(Martin et al., 2012). Monthly WeMOi data were 
downloaded from crudata.uea.ac.uk/cru/data/moi/
Web_WeMOi-2020.txt. Pearson’s product moment 
correlations were used to assess the relationship 
between zooplankton DM and Chl a, and DM and 
WeMOi using time series of monthly data. Additional 
analyses considered a time lag of one month for 
zooplankton DM. The annual WeMOi is the average 
value of the corresponding monthly WeMOi (Martin 
et al., 2012).
Jellyfish blooms were defined according to the 
methodology described by Pestorić et al. (2021). 
During the study period, the following macrojellyfish 
species (Cnidaria, Scyphomedusae: Aurelia solida, 
Chrysaora hysoscella, Cotylorhiza tuberculata, Pela-
gia noctiluca, Rhizostoma pulmo and Ctenophora, Lo-
bata: Mnemiopsis leidyi) were found to be blooming 
in the Gulf of Trieste (Kogovšek et al., 2010; Pestorić 
et al., 2021, own observations). The annual jellyfish 
bloom index (Jbi) was calculated as the cumulative 
number of months in a year in which macrojellyfish 
blooms occurred in the study area. The non-parametric 
Kruskal–Wallis test and Dunn’s post-hoc multiple pair-
wise comparison test were used to determine whether 
there were significant differences in zooplankton DM 
results between years with different JBi. Hierarchical 
clustering and non-metric multidimensional scaling 
(NMDS) were used to visualise how the different years 
clustered based on the Bray-Curtis dissimilarities of 
the DM influenced by WeMOi and Jbi.
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Jan MALEJ et al.: LONG-TERM STUDY OF ZOOPLANKTON BIOMASS IN THE SOUTH-EASTERN PART OF THE GULF OF TRIESTE (ADRIATIC SEA), 133–144
RESULTS
Fig. 2 shows all zooplankton DM (318 results) in 
the study period 1974–2019, while the descriptive sta-
tistics with mean, geometric mean, median, standard 
error and deviation are shown in Table 1.
Overall, zooplankton DM varied widely, from 
less than 10 mg m-3 (23% of results) to 99.3 mg m-3, 
with 9% of results above the 90th percentile. CoV 
was slightly higher in the periods before 2004 (0.77 
for 1974–1980, 0.74 for 1989–2002) and was lowest 
in the latter period (0.61). In the years 1989–2002, 
DM was frequently above the overall median (67% 
of results) and the mean (51% of results). In contrast, 
high results were less frequent in the years 1974-1980 
and after 2004. In particular, the periods between 
2004 and 2010 and after 2014 were characterised by 
low DM (66% of results were below the median and 
86% below the overall mean).
Interannual and seasonal changes in zooplankton DM
The interannual variations in zooplankton DM 
were considerable (Figs. 2 and 3). Years with sta-
tistically significant (p ≤ 0.01) lower DM values 
were observed before 1980 (except for 1979) and 
after 2004 (with the exception of 2012, 2014). The 
years with the highest annual CoV (> 0.8) were 
1993–1997, 2012 and 2017, while the DM results 
fluctuated the least in 2004–2009 and 2014–2016 
(CoV < 0.5). The annual fluctuations in zooplankton 
DM can be clearly visualised by calculating the 
normalised annual anomalies (Fig. 3): the posi-
tive anomalies are concentrated in the years 1989 
to 2002, followed by the conspicuous negative 
anomalies from 2004 onwards.
Tab 1: Descriptive statistics for zooplankton dry mass 
results in the period 1974–2018.
Tab. 1: Opisna statistika za rezultate zooplanktonske 
suhe mase v obdobju 1974–2018.
Fig. 2: Scatter plot of zooplankton dry mass (mg m-3) over the study period (1974–2019), the median (full 
yellow line) and the 10th and 90th percentiles (dashed green lines). The different seasons are marked with 
different colours (winter – dark blue, spring – orange, summer – green, autumn – light blue).
Sl. 2: Diagram suhe mase zooplanktona (mg m-3) v preiskovanem obdobju (1974–2019) z mediano (rumena 
linija) ter 10. in 90. percentilom (črtkani zeleni liniji). Sezone so označene z različnimi barvami (zima – temno 
modra, pomlad – oranžna, poletje – zelena, jesen – svetlo modra). 
Dry Mass (mg/m3)
Mean 22.4
Geometric Mean 17.1
Standard Error 1.0
Median 16.2
Standard Deviation 18.2
Sample Variance 332.8
10th Percentile 7.3
90th Percentile 49.8
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The average annual cycle of zooplankton DM 
throughout the study period showed higher biomass 
but also greater variability in the warmer season 
(May–September) (Fig. 4), while values were lower 
in the colder months (December–April). The annual 
DM maxima were most frequently measured in May 
(44 % of all years), followed by September (16 %).
Comparison with Chl a, WeMOi and 
jellyfish blooms (Jbi)
Due to gaps in sampling and the lower frequency 
of sample collection, we decided to conduct further 
analyses and examine DM relationships to phyto-
plankton biomass and climate index only for results 
since 1993. Chl a fluctuated widely with maximum 
values above 10 mg m-3 (10.15 and 12.06 mg m-3 in 
November 1996 and February 1999, respectively), 
while the lowest value was measured in June 2018 
(0.16 mg m-3). In addition, the entire period after 
2006 was characterised by lower Chl a with annual 
mean values below 1.0 mg m-3, in contrast to the 
previous years with mean values of 1.07 to 2.11 
mg m-3. 
The monthly WeMOi showed positive and negative 
values with the highest monthly value of 3.19 (April 
1994) and the lowest –3.18 (June 2006). While a 
considerable part of the WeMOi was neutral (values 
between –1.00 and 1.00), the positive WeMO phase 
prevailed in the 1990s and early 2000s and the nega-
tive WeMO from 2002 onwards.
Correlations between zooplankton DM, Chl a 
and WeMOi were analysed using monthly data from 
1993–2018. Although we found positive correlations 
between zooplankton DM and Chl a as well as DM and 
the positive phase of WeMOi, these were weak (0.131 
and 0.151, respectively) and not significant. Reanalysis 
with a time lag did not improve the correlation.
Analysis of the annual WeMOi time series indicated 
that before 2002 the positive WeMO phase prevailed 
(Fig. 5). It is associated with higher rainfall and river 
flows as well as lower sea temperatures, conditions 
that are considered favourable for overall biological 
productivity (Martin et al., 2012).
Throughout the study period, there were years 
without massive jellyfish blooms and/or with short-
lived seasonal blooms of one or two species (Jbi ≤ 3) 
as well as years in which several species bloomed over 
a longer period and at different times of the year. Typi-
cally, the blooms of A. solida occurred in winter-spring, 
C. tuberculata in summer, R. pulmo in autumn-winter-
spring and since 2016 M. leidyi in summer-autumn. 
Fig. 3: Normalized annual anomalies of zooplankton dry mass during the study period (1974–2018).
Sl. 3: Normalizirane letne anomalije zooplanktonske suhe mase v obdobju 1974–2018.
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The annual Jbi varied from 0 (no bloom in one year) to 
10 (ten months with bloom in one year). Until 2004, 
there were nine years without blooms, moreover, the 
Jbi was lower in the years 2009–2014 (≤ 5) than in the 
years 2004–2008 and 2015–2019 (Jbi 6-10). 
The Kruskal–Wallis test (p = 0.0016) and Dunn’s 
post-hoc multiple pairwise comparison test (p = 
0.0016) revealed significant differences in zooplankton 
DM between the years without blooms and the years 
with Jbi ≤ 3 and those with multi-species blooms over 
a longer period (Jbi ≥ 4). Hierarchical clustering and 
nMDS ordination of DM showed that WeMOi and 
Jbi affected DM in different ways and their influence 
varied across years. While a higher Jbi was associated 
with a lower DM (years from 2004 onwards), a higher 
DM was generally related to the positive WeMOi phase 
(1980s and 1990s).
DISCUSSION AND CONCLUSIONS
Many marine organisms that are critical to the 
functioning of the ecosystem and to humans, such as 
exploited fish, shellfish, squid, some marine mam-
mals, seabirds and sea turtles, to name but a few, are 
affected by the quantity of zooplankton. Therefore, 
zooplankton biomass can be considered a key indi-
cator of changes due to environmental and anthro-
pogenic pressures, including responses to climate 
change. Zooplankton data are also important for the 
effective implementation of the MSFD (Gorokhova et 
al., 2016), particularly for the development of methods 
to assess biodiversity in pelagic habitats (Descriptor 1) 
and food webs (Descriptor 4). Especially in the Medi-
terranean Sea, where zooplankton-based indicators 
are still in the development phase, long-term data are 
important to define the baseline situation and capture 
trends and shifts (Francé et al., 2023).
Among the various methods used to determine 
zooplankton biomass, the dry mass procedure has 
been evaluated and standardised since the seminal 
work of Lovegrove (1966). Although there are many 
papers on various aspects of zooplankton in the 
northern Adriatic (Pearson et al., 2021), few report 
on biomass, especially DM. In our article, we present 
the results of more than four decades of monitoring 
zooplankton biomass in the Gulf of Trieste using this 
standardised method. Furthermore, the very similar 
mesh size (200 – 250 µm) of the zooplankton nets 
used and the standardised determination of DM at 600 
C ensure that our dataset provides comparable results 
over the period from 1974 to 2019.
The results of zooplankton biomass measurements 
of the Adriatic Sea during 1971–1981 were summarised 
in an article by Benović et al. (1984). These authors 
reported significant differences in zooplankton DM, 
with results from the northern Adriatic, including the 
Gulf of Trieste, being higher than in other coastal areas 
and open Adriatic waters. The range of DM values in 
Fig. 4: Mean seasonal pattern (1974–2018) of zooplankton dry mass (mg m-3) (red line) and the 10th and 90th 
percentiles (grey lines).
Sl. 4: Sezonskost povprečne (1974–2018) zooplanktonske suhe mase (mg m-3) (rdeča linija) z 10. in 90. 
percentilom (sivi liniji).
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the Gulf of Trieste was between less than 10 and over 
100 mg m-3 (1.2 – 177.2 mg m-3) and the mean value 
was 18.5 mg m-3. The largest proportion of results (36.7 
%) was between 10 – 20 mg m-3 with the highest values 
measured in spring and the lowest in winter. The ob-
served seasonal dynamics was consistent with ours, but 
our mean DM value over the entire period 1974–2019 
was thus higher (22.4 mg m-3, Tab. 1). However, we 
found a significant difference between the period 
before and after 2003 (mean 29.5 vs. 16.0 mg m-3, 
respectively). This decline in zooplankton DM in the 
Gulf of Trieste is consistent with the findings of Mozetič 
et al. (2012), who reported a regime shift in plankton 
between 2002 and 2003. This shift was related to lower 
nutrient concentrations due to reduced freshwater 
inputs from the Soča River in 2002–2007, particularly 
in 2003, a decline in seasonal diatom blooms and the 
dominance of smaller-sized phytoplankton.
A study (Kamburska & Fonda Umani, 2009) focus-
sing on the temporal pattern of net zooplankton at a 
coastal station in the western part of the Gulf of Trieste 
covered two decades of monthly sampling (1986–2005) 
and was complemented by some earlier data. The DM 
values were in a similar range to ours (1 – 95 mg m-3), 
and like us they found the highest monthly average in 
May and lower from November to February. In contrast 
to our observations of decreasing DM after 2003, they 
reported higher DM in the last two years of their study 
(2004, 2005 only until September), while their values 
in the 1990s were similar to ours. Nevertheless, the 
work of Benović et al. (1984), Kamburska & Fonda 
Umani (2009) and ours indicate a large seasonal and 
interannual variability of zooplankton DM, similar 
seasonal dynamics and a comparable range of DM 
values. However, in the last period of our study (after 
2003), which was not covered by previous reports, we 
found a significantly lower zooplankton DM. Despite 
some interannual differences, this decline is consistent 
with general observations of oligotrophication in the 
northern Adriatic (Brush et al., 2021). 
Despite numerous studies on zooplankton com-
munities, the estimated DM values of samples col-
lected using nets with a comparable mesh size to ours 
(200–250 µm) have rarely been reported from other 
Mediterranean regions. A ten-year study (2007–2017) 
in the western Mediterranean (Fernández de Puelles 
et al., 2023) showed considerable variability without 
clear temporal trends, with higher DM values (> 10 
mg m-3) in the western Alborán Sea and lower values 
(< 5 mg m-3) in the eastern Balearic Sea. In an earlier 
Fig. 5: Annual WeMOi (Western Mediterranean Oscillation index) from 1974 to 2019: red bars – positive 
WeMO phase, blue bars – negative WeMO phase.
Sl. 5: Letni indeks WeMO (Zahodna sredozemska oscilacija) v letih od 1974 do 2019: rdeči stolpci – pozitivna 
WeMO faza, modri stolpci – negativna WeMO faza.
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study (1994–2003) in the Balearic Sea, Fernández de 
Puelles & Molinero (2008) also reported similarly low 
DM values with an average of 5.4 mg m-3. Average an-
nual DM values between 1.43 and 3.56 mg m-3 were 
found off Marseille, with the biomass being highest at 
the nearshore stations (Gaudy & Champalbert, 1998). 
Similar DM values (0.7 – 5.2 mg m-3) were found in 
the coastal area of Cyprus, while higher DM values 
were measured in the Saronikos Gulf (up to 11.9 mg 
m-3 on average) and in the north-eastern Aegean Sea 
(Hannides et al., 2015). Lakkis (1990) reported values 
(2 – 20 mg m-3) in the Lebanese coastal waters. These 
comparisons show that despite oligotrophication in 
recent decades, zooplankton DM in the Gulf of Trieste 
is still higher than in most Mediterranean regions.
In our analysis of monthly data, no clear relation-
ship was found between the regional atmospheric 
index WeMOi and zooplankton DM, although a weak 
positive correlation was established, i.e. positive 
WeMOi were associated with higher DM. Despite 
the non-significant correlation, years with lower 
DM showed a higher frequency of negative WeMOi 
monthly values below - 2.00, which are assumed to 
have a stronger influence on dry conditions (Martín 
et al., 2012). Accordingly, Milošević et al. (2016), 
who studied seasonal and annual precipitation 
(1963–2012) in Slovenia, found that a positive WeMO 
phase led to more precipitation, especially in spring 
and autumn, which could have an impact on local 
river discharge. 
In the northern Adriatic, a negative WeMO phase 
is associated with dry and stable weather with less 
precipitation and lower river discharge. In addition, 
higher surface temperatures during stronger and longer 
negative WeMO phase lead to stronger thermal strati-
fication, which reduces the mixing of nutrients in the 
surface layers. While the analysis of the relationship be-
tween the monthly WeMOi data and zooplankton DM 
did not provide conclusive results, the annual WeMOi 
time series (1974 – 2019) showed a clear difference 
between the period before 2002, when the positive 
phase prevailed and zooplankton DM was higher, and 
the more recent period with negative WeMO phase 
and lower DM (Fig. 5).
Martín et al. (2012) found a clear link between 
climate fluctuations, i.e. positive WeMOi values, and 
higher sardine and anchovy production. They suggest-
ed that WeMOi impacted fish populations indirectly 
through effects on local environmental factors such 
as temperature and river discharge and referred to the 
positive phase of WeMO as a “cool waters, high river 
runoff phase”. Their conclusion was that “a regional 
climatic index such as the WeMOi is more closely 
linked to local environmental conditions and explains 
a higher proportion of the variation in fisheries produc-
tivity than other climatic indices commonly used for 
the Atlantic Ocean, such as the NAOi”. 
Based on a 13-year study in the northwestern 
Mediterranean (Bay of Calvi), Fullgrabe et al. (2020) 
found a correlation between the winter NAO and the 
spring zooplankton peak and hypothesised an influ-
ence of large-scale processes on the regional zoo-
plankton. In contrast, Feuilloley et al. (2022) found 
no significant correlation with NAOi and WeMOi 
in their long-term zooplankton study (1995–2019), 
which was also conducted in the northwestern Medi-
terranean (entrance to Villefranche Bay) and pointed 
to the importance of biotic interactions. Piontkovski 
et al. (2011) investigated the effects of atmospheric 
anomalies on zooplankton communities in the 
northern Adriatic and Black Sea using the NAO as an 
example and found that while some species showed 
a correlation with the NAO, changes in integrative 
characteristics such as DM were not pronounced. 
Berline et al. (2012) compared six mesozooplankton 
time series from 1957–2006, including the coastal 
location in the western Gulf of Trieste. They found no 
significant correlations between the climate indices 
and pointed out that local factors dominate at these 
coastal stations. However, they suggested that the 
link between local and large-scale climate should 
be further investigated if we want to understand the 
fluctuations in zooplankton.
Our analysis of 25 years (1993–2018) of paired 
phytoplankton and zooplankton samples showed a 
weak and statistically non-significant positive correla-
tion between Chl a and zooplankton DM. This lack 
of strong coupling may be related to several reasons. 
Both biomass measurements are integrative and do not 
provide information on taxonomic composition and 
specific feeding relationships. Furthermore, there may 
be a different time lag between primary producers and 
consumers at different times of the year, in addition to 
possible predatory control of zooplankton. 
Jellyfish can exert a strong top-down control on 
other zooplankton (Schneider & Behrends, 1998). The 
studies suggest that the effects are potentially large but 
show strong spatial and temporal variation depending 
on the occurrence of the blooms (Stoltenberg et al., 
2021). Most studies on the control of zooplankton 
by jellyfish in European seas come from the enclosed 
marine systems such as the Baltic Sea and the Black 
Sea. However, Stoltenberg et al. (2021), who reviewed 
the trophic interactions of jellyfish in the Baltic Sea, 
conclude that the lack of spatially and temporally 
consistent data on both jellyfish and their prey com-
munities does not currently allow for a more holistic 
and quantitative assessment of the impact of jellyfish 
on mesozooplankton.
We compared the frequency of jellyfish blooms 
and zooplankton DM in the northernmost gulf of the 
enclosed Adriatic Sea. Statistical tests showed that DM 
was significantly lower in years when jellyfish blooms 
of several species occurred over longer periods of time. 
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A similar conclusion was reached by Malej (1989), 
who studied zooplankton DM and community compo-
sition in three years, one of which was characterised 
by a prolonged Pelagia noctiluca bloom; the “Pelagia 
year” had the lowest DM. A more general conclusion 
can be drawn from the study by Wright et al. (2021). 
Despite the paucity of data on global jellyfish biomass, 
they used a global biogeochemical model to suggest 
that jellyfish play an important role in regulating global 
marine plankton ecosystems which has generally been 
neglected. 
In summary, we report the results of zooplankton 
biomass over more than four decades in the Gulf of 
Trieste using a standardised method to determine dry 
mass. The use of nets with similar mesh sizes (200 – 
250 µm) ensures that our dataset provides comparable 
results for the period 1974–2019. The range of DM 
values was large (< 10 to almost 100 mg m-3), we found 
a significant difference between the period before 
and after 2002, and the decline in the last period was 
consistent with the regime shift in plankton observed 
between 2002 and 2003 by Mozetič et al. (2012). 
Although the correlations between monthly regional 
climatic index WeMOi, Chlorophyll a and zooplankton 
DM were not significant (p > 0.05), they were weakly 
positively correlated. Moreover, the period with lower 
zooplankton DM (after 2002) was characterised by 
the predominance of the negative WeMO phase. In 
addition, the statistical tests showed that DM values 
were significantly lower in years when several jellyfish 
species bloomed over longer periods.
ACKNOWLEDGEMENTS
The authors acknowledge the financial support 
from the Slovenian Research and Innovation Agency 
(research core funding No. P1-0237 “Coastal Sea Re-
search”) and national monitoring programme financed 
by the Slovenian Environment Agency of the Ministry 
of Environment and Spatial Planning.
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DOLGOROČNA ŠTUDIJA ZOOPLANKTONSKE BIOMASE V TRŽAŠKEM ZALIVU 
(JADRANSKO MORJE)
Jan MALEJ
Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia
e-mail: jan@malej.si
Tjaša KOGOVŠEK
Center for Marine Research, Rudjer Bošković Institute, G. Paliaga 5, HR-52210 Rovinj, Croatia
Martin VODOPIVEC, Janja FRANCÉ & Patricija MOZETIČ
Marine Biology Station Piran, National Institute of Biology, Fornače 41, SI-6330 Piran, Slovenia
Matevž MALEJ
Faculty of Management, University of Primorska, Izolska vrata 2, SI- 6000 Koper, Slovenia
Alenka MALEJ
Marine Biology Station Piran, National Institute of Biology, Fornače 41, SI-6330 Piran, Slovenia
e-mail: alenka.malej@nib.si
POVZETEK
V prispevku poročamo o rezultatih zooplanktonske biomase v Tržaškem zalivu, ki smo jo merili z upo-
rabo standardizirane metode za suho maso v več kot štirih desetletjih. V tem obdobju (1974–2019) smo 
za vzorčenje zooplanktona uporabljali mreže s primerljivo velikostjo okenc (200 – 250 µm) kar zagotavlja 
primerljivost meritev. Izmerjene vrednosti suhe mase so nihale v širokem razponu (< 10 do blizu 100 mg 
m-3), ugotovili smo statistično značilne razlike med obdobji pred in po letu 2003, upad biomase v zadnjem 
obdobju pa je bil skladen z ugotovitvami spremenjenega režima v planktonu med 2002 in 2003. Čeprav 
korelacija med regionalnim atmosferskim indeksom (WeMOi), klorofilno biomaso in zooplanktonsko suho 
maso ni bila statistično značilna, so bile višje vrednosti biomase povezane s pozitivno fazo WeMO in višjimi 
vrednostmi klorofila. Statistični testi pa so pokazali značilno nižjo biomaso zooplanktona v letih, ko smo 
beležili masovno pojavljanje več vrst želatinastega planktona, ki so trajali daljša obdobja.
Ključne besede: mrežni zooplankton, suha masa, časovni niz, klorofil a, masovno pojavljanje meduz
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