original scientific paper UD C 504.4:59(262.3-17) 626/627:504.4 BIOLOGICA L FILTER STATIONS : A NEW ARTIFICIA L REEF CONCEP T T O COMBA T TH E EFFECTS O F EUTROPHICATiO N IN COASTA L SEAS Michael STACHOWIT5CH Institute of Zoology, University of Vienna, A-109Q Vienna, Aithanstrasse 14 ABSTRACT Shallow coastal waters are the most important marine ecosystems, both ecologically and economically. They are also the most endangered, with eutrophication increasingly being recognized as one of the greatest threats. The pre­sent contribution proposes to combat not only the major symptoms of eutrophication - decreased water transpar­ency, excessive alga! blooms, mucus production, and oxygen deficiency - but also the causes by utilizing the natural filter-feeding capacity of marine organisms to remove material suspended in the water. The concept involves provid­ing optimal structures on which these organisms can settle and grow. In the North Adriatic Sea, this fauna includes ascidians, bivalves, sponges, and tube worms which function as a natural eutrophication control. This low-tech so­lution has been developed and tested in the Gulf of Trieste and is patented. It involves inexpensive, miniature "biological filter stations" with many advantages over traditional artificial reef structures. It is the only conceivable strategy that can improve the quality of the marine environment once nutrients and other pollutants have entered the sea. Finally, it also contributes directly to restoring original benthic community structure and function: the over­grown structures are ultimately indistinguishable from the aggregations of sessile organisms that characterize the sublittoral soft bottoms of the North Adriatic Sea. Key words: artificial reefs, eutrophication, anoxia, phytoplankton blooms, marine snow, benthos, recolonization, North Adriatic Sea THE PROBLEM Shallow coastal waters are the most important marine ecosystems, both ecologically and economically. As opposed to the open ocean, the biomass, diversity, and productivity of the flora and fauna is higher here. The world's main fisheries, including virtually all mariculture efforts, are therefore concentrated in such shallow shelf seas. They are also the most endangered due to coastal and shoreline engineering measures, intense exploitation of mineral (e.g., oil) and biological resources (fisheries, mariculture), unabated input of toxic substances, and rising pressures from tourism. Increasingly, eutrophicati­on - essentially an over-enrichment with nutrients - is being recognized as one of the greatest threats, with the exact role of anthropogenic or "cultural" eutrophication still being debated. The symptoms include reduced water transparency, excessive algal blooms, mucus production ("marine snow"), oxygen deficiency, and mass mortality of benthic organisms. These developments have been registered in many shallow coastal areas, including the North Sea (Rachor, 1985), Baltic Sea (Larson ef a/., 1985), Scandinavian waters (Rosenberg, 1985), Japan (Imaba­yashi, 1983), and the U.S.A. (Officer etal, 1984). Eutro­phication is also an immediate concern in the Medi­terranean Sea (UNESCO, 1988), with a particularly acute situation in the Noithern Adriatic Sea (Brambati, 1988). The primary result of increased nutrient input is the stimulation of plant growth. In the sea this involves sea­weed growth and phytoplankton blooms. The former can be a nuisance in certain semi-enclosed systems (e.g., Ulva growth in the Venice Lagoon), but cannot be considered to be a chief threat to the ecosystem as a whole. Abnormal phytoplankton blooms and marine snow events, on the other hand, are known to be directly responsible for a wide range of negative impacts. These range from relatively harmless effects such as clogged fishing nets and un-sightly foam deposits on beaches, to major threats to marine life (red tides) and to human health by toxic algal blooms that cause ciguatera, Midiael STACHOWITSCH: BIOLOGICAL FILTER STATIONS: A NEW ARTIFICIAL REEf CONCEPT T O COMBAT..., 7-14 Fig. 1: Test of Schizaster canatiferus with early stage of overgrowth. Note that tube worms (Pomatoceros tri­queter and Serpula sp.) as well as hydrozoan colonies originate from lower surface (from Nebelsick et al., 1997X25 m depth, Gulf of Trieste). All photos: M. Stachowitsch. SI. 1: Lupina qjtirskega ježka Schizaster canaliferus v zgodnji fazi zaraščenosti. Zanimivo je, da cevkarji (Po­matoceros triqueter in Serpula sp.) in kolonije trdo­živnjakov izvirajo iz spodnje površine lupine (povzeto po Nebelsick et al., 1997) (Tržaški zaliv v globini 25 m). Vse fotografije: M. Stachowitsch. amnesic shellfish poisoning (ASP), paralytic shellfish poisoning (PSP), diarrhoeic shellfish poisoning (DSP), and neurotoxic shellfish poisoning (NSP) (for a recent review, see Richardson, 1997). At the same time, ho­wever, eutrophication can lead to the destabilization and collapse of entire marine ecosystems. This process typically involves the collapse of plankton populations, which then sink to the sea floor. Here, bacterial de­composition consumes large amounts of oxygen, lead­ing to hypoxia or anoxia in the bottom water layers. In the Adriatic Sea, for example, the negative impact of settling plankton blooms is compounded by the "mare sporco" phenomenon: mucus material in the water co­lumn and large-scale mats on the surface. This pheno­menon has been known since historic times (Fonda-Uma­ni eta!., 1989), but appears to be increasing in frequency and severity. Mucus in the form of less conspicuous "marine snow" is common in the Northern Adriatic Sea and, indeed, worldwide. In the Adriatic, marine snow is acknowledged as being produced by diatoms in the pelagic zone, i.e., as being of phytopiankton origin: benthic diatom biomass is insufficiently high to produce the quantities of mucus aggregates observed in 1988 and 1989 (Herndl etai, 1990). Recently, five types or stages of this material have been distinguished: 1. macroflocs, 2. stringers, Fig. 2: Early overgrowth stage on the bivalve Pecten jacobaeus consisting largely of dense serpulid tube worms; growth originates from lower surface of shell. Note large, sediment-covered and epigrowth-free top side of valve. SI. 2: Zgodnja faza zaraščenosti na Jakobovi pokrovači Pecten jacobaeus, ki je v veliki meri posledica rasti številnih cevkarskih mnogoščetincev iz spodnje površi­ne školjke. Zanimiva je velika, z usedlinami prekrita gornja stran pokrova školjke brez obrasti. 3. clouds, 4. creamy surface layers, and 5. gelatinous surface layers. This typology is based on size, shape, stability, be­havior, and effect on benthos (Stachowitsch et al., 1990). Such "mare sporco" events pose a serious eco­logical and economical threat: they accelerate the dete­rioration of the ecosystem and have a severe negative impact on the fishing and tourism industry. TRADITIONAL SOLUTIONS Mucilage and other effects of eutrophication can only be combated by a multi-pronged strategy. First and fore­most is the "beginning-of-the-tube" approach: nutrient inputs into the environment must be reduced. Secondly, "middle-of-the-tube" measures can be implemented. This can involve the restoration of river meanders, of river banks, and of marshlands and estuaries, or the promotion of algal growth in retaining basins before waste water reaches the sea. As opposed to the above long-term ap­proaches, useful, short-term, "end-of-t he-tube" measures are more difficult to envision in the sea. Indeed, no tec­hnological solution is conceivable that could remove nutrients and/or the resulting phytopiankton blooms once waste waters have entered the sea. In the Adriatic and elsewhere, the applied in situ measures have been restric­ted to cosmetic activities such as installing artificial barri­ers against floating mucus mats or removing seaweed. Michael STACHOWITSCH: BIOLOGICAL FILTER STATIONS: A NEW ARTIFICIAL REEF CONCEPT T O COMBAT..., 7-1-4 Fig. 3: Lower surface of horizontally positioned 25 x 25 cm asbestos cement plates in 25 m depth. Overgrowth patterns on artificial substrates confirm those observed on biogenic structures. Dense early-stage epigrowth consists of tube worms, colonial ascidians, bivalves, and hydrozoans. Note sea urchin Psammecbinus micro­tuberculatus on lower left and metal rod (bottom mid­dle) holding substrate in place 1m above the bottom. (25 m depth, Gulf of Trieste) SI. 3: Spodnja površina vodoravno položenih 25 x 25 cm plošč iz azbestnega cementa v globini 25 m. Vzorci za­ rasti na umetnih podlagah so enaki vzorcem, opazo­ vanim na biogenskih strukturah. Gosta zarast v zgodnji fazi sestoji iz mnogoščetincev, kolonijskih kozolnjakov, školjk in trdoživnjakov. Zanimivo je, kako mali morski ježek (Psammecbinus microtuberculatus) spodaj levo in kovinska palica (spodaj v sredini) držita podlago skupaj kak meter nad dnom, (Tržaški zaliv v globini 25 m) One traditional approach to ameliorating damage to marine ecosystems is to introduce artificial structures into the sea. The great interest in such artificial reefs is reflected in recent bibliographies containing several thousand references (Stanton ef a!., 1985; Reeff & Mc Gurrin, 1986). Such reefs have been employed in the Adriatic Sea of itaiy (Bombace, 1989} and proposals have been made for the Slovene part of the Adriatic as well (Fonda, 1 995). The term artificial reef is used to describe benthic structures created accidentally or deliberately by human activities. The more general term, artificial habi­tat, refers to structures deployed either on or above the sea floor, including floating or midwater fish-aggregating devi ces (FADs) (Bohnsack et al., 1991), 1 he topic has gained public interest in recent years due to the attempts of the oil industry to abandon or discard their decommissioned oil platform-related structures at sea under the guise of habitat improvement through artificial reefs. The materials used to construct artificial habitats include concrete, iron and steel, reinforced concrete, ce­ramic, various plastics or plastic concrete, and a wide range of so-called "materials of opportunity" (automobile tires and bodies, derelict ships) (Grove ef al., 1991). Overall design also varies widely, with advanced struc­tures having complex modular forms requiring assembly on land or in the water. A recent, global overview of ail aspects of artificial habitats is provided in Seaman & Sprague (1991). The common structural feature of virtually all these artificial reefs is their large size and enormous weight. Similarly, the common biological feature is that most are ultimately deployed to attract fishes. Fishes, however, represent only one of many compartments of the marine ecosystem, and traditional artificial reefs can therefore contribute only little to restoring the original community structure of damaged shallow seas or actually counter­acting damaging influences. A NEW APPROACH The present proposal is a low-tech biologies! strategy that both combats eLitrophication-related effects in the sea and promotes the overall recolonizatiori process of the sea floor. It represents a short- to medium-term mea­sure and utilizes the natural filter-feeding capacity of ma­rine organisms to remove excess biomass - bacteria, phy­toplankton, marine snow, detritus, etc. - from the water. The macroepibenthos in the Adriatic Sea consists largely of filter- or suspension-feeding organisms and in­cludes sponges, bivalves, ascidians, tube worms, brittle stars, hydrozoans, and bryozoans. This benthic fauna is' capable of removing enormous amounts of material from the water column. Depending on water depth, intact benthic communities can potentially filter the entire vo­lume of a basin within days or weeks (3 d, Laholm Bay, 10 m, Loo & Rosenberg, 1989; 4-6 d, Oost.erschelde, 5-35 m, Smaal ef al., i 986; 20 d, Gulf of Trieste, 25 m, Ott & Fedra, 1977). By regulating the processes in the overlying water, they stabilize the entire system. For example, such communities are considered to control water quality in the Bay of Brest (Hily, 1991) and to stabilize the estuarine ecosystem of the Oosterschelde, Netherlands (Herman & Scholten, 1990). in San Francisco Bay, benthic filter feeders have been termed a "natural eutrophication control" (Officer eta/., 1982). In principle, this mecha­nism involves a conversion of pelagic biomass into benthic biomass, the latter having a lower respiration per unit weight. The benthos functions as a battery in which large, perennial species take up excess pelagic material and store it in the form of body tissue etc. (Ott, 1981). This is important for the energetics and oxygen balance of the ecosystem. This feature gains additional signifi­cance in eiitrophicated waters, where an intact fauna can dampen the effect of iarge nutrient loading; at the same time, the overall system is very sensitive to fluctuations in the benthic populations themselves (Herman & Scholten, 1990). It is precisely these communities that are, ho­ Miclw i STACHOWITSCH : BfOLOGICA l FILTER STATIONS'. A NEW ARTIFICIAL REEF CONCEP T T O COMBAT..., 7-14 Fig. 4: Upper surface of horizontally positioned 25x25 cm asbestos cement plate. Late-stage situation with thick sediment layer on upper surface; epigrowth con­sisting of tube worms and hydrozoan colonies (over­grown by colonial ascidians) growing from lower side up to edges of substrate. Si. 4: Gornja površina vodoravno položene 25 x 25 cm plošče iz azbestnega cementa. Stanje iz pozne faze z debelo plastjo usedlin na gornji površini; obrast, ki sestoji iz cevkarjev in kolonij trdoživnjakov (preraslih s kolonijskimi kozolnjaki), raste iz spodnje strani navzgor do robov podlage. wever, most strongly affected by anoxia (Stachowitsch, 1984; 1988). The short-term disturbances in the pelagic subsystem cause iong-ierm disturbances in the benthic subsystem, making the latter the "memory" of ecosystem collapses (Stachowitsch, 1992). The present proposal involves supporting filter feed­ers by providing them with optimal structures on which to settle and grow. The larvae of these organisms are naturally present in the water and eventually settle on available hard structures. Such substrates are generally in limited supply on the mud or sand bottoms character­izing shallow coastal seas. In the soft bottoms of the North Adriatic Sea they include a wide range of biogenic structures, mostly dead bivalve shells (e.g., Chlamys spp., Area noae, Pecten jacobaeus, Acantho­cardium sp., Laevicardium oblongum) and gastropod shells fe.g., Apoirhais pes-pelecani, Murex brandaris, Truncuiariopsis trunculus). Epigrowth on the latter typi­cally takes place during the phase in which they are oc­cupied by hermit crabs (Stachowitsch, 1980), Other structures include the shells of the partially embedded, vertically oriented large bivalves Pinna spp. and tests of the irregular sea urchin Sc.hizaster canaliferus (Nebelsick et at., 1997), which emerge and die in large numbers during oxygen crises. Twenty-five years of field research in the Northern Adriatic Sea have yielded valuable information on the ! Fig. 5: Vertically oriented series of asbestos cement panels. Dense epigrowth of tube worms and ascidians on all sides o f panels. Higher panels have more growth and also serve as substrate for egg cases of the cepha­lopod Loligo vulgaris. Overall height, ca. 2 m. (25 m depth, Gulf of Trieste) SI. 5: Navpično postavljene plošče iz azbestnega ce­menta. Gosta obrasf cevkarjev in kolonijskih kozol­njakov na vseh straneh plošč. Višje plošče so bolj za­rasle in so hkrati tudi podlaga za jajčeca navadnega lig­nja (Loligo vulgaris). Skupna višina pribl. 2 m. (Tržaški zaliv v globini 25 m) conditions necessary for the development, of epigrowth on the above secondary hard bottoms. Small, flatter substrates (small bivalves, shell fragments) are subopti­mal. Fresh, cleaned bivalve shells and (sealed) gastro­pod shells, for example, were manipulated, overturned and scattered by hermit crabs, sea stars and holothurians and tended to be rapidly covered by sediment (Stachowitsch, 1979; Fig. 5 in Stachowitsch & Fuchs, 1995). Despite their fragility, the somewhat larger, rounded Sc.hizaster canaliferus tests are better suited. After mass mortalities, the white S. canaliferus tests are among the most conspicuous and abundant substrates on the sediment surface. Numerous in situ observations show that epigrowth typically initiates on the bottom sides of S. canaliferus, probably due to the influence of Midiael STACHOWITSCH : BIOLOGICAL FILTER STATIONS: A NEW ARTIFICIAL REEf CONCEP T T O COMBAT..., 7-14 Q Fig. 6: Schematic illustration of prototype underwater structure consisting of buoyant element (1), anchoring element (3) and flexible connection (2; not draw to scale). Fist-sized device unfurls automatically when released and lands correctly oriented on the bottom. Overall height ca. 1 m. $1. 6: Shematski prikaz podvodnega prototipa, ki ga sestavljajo plavajoči element (1), sidro (3) in prožna povezava (2; ni v merilu!). Napravica, velika kot pest. se ob sprožitvi odvije in v pravilnem položaju pristane na morskem dnu. Skupna višina pribl. 1 m. sedimentation (e.g., accumulation of sediment in the ambulacra! grooves; Fig. 1). This growth pattern was also evident on the larger bivalve Pecten jacobaeus (Fig. 2), whose population exploded after a mortality event in 1984, The pattern was further confirmed by artificial substrate experiments: fouling organisms on horizontal asbestos cement plates (25x25 cm) suspended 1 m above the bottom (25 m depth) were largely restricted to the undersurface (Fig. 3). The top sides were rapidly covered by a thick layer of sediment (Fig. 4). A reversal of this pattern (i.e. no influence of sedimentation, growth on the upper surface) is achieved by hermit crab-occupied gastropod shells (Stachowitsch, 1979, 1980): the crabs' movements keep the shells off the sea floor, free of sediment and predators/grazers, and guarantee a stable orientation. The established organisms on such shells are known to survive after the shell is abandoned by the crab, leading to the aggregations of sessile invertebrates (so-called multi-species clumps) that characterize this benthic community. Larger artificial structures consisting of series of vertically oriented asbestos cement plates were rapidly encrusted on all sides with dense epigrowth (Fig. 5); the higher plates exhibited more growth, which may be related to strong vertical gradients in bottom water oxygen concentrations, with values increasing rapidly further above the sediment surface (Malej & Malačič, 1995). Fig. 7: Overview of an overgrown underwater "bio­logical filter station". Such structures accelerate recolo­nization of the sea floor and help stabilize the entire eco­system. Note comparatively sparse epigrowth on surr­ounding sediment bottom (25 m depth, Gulf of Trieste). SI. 7: Pogled na podvodno "biološko precejevalno po­stajo". Takšne zarasle strukture pospešujejo ponovno poseljevanje morskega dna in pomagajo pri stabilizaciji celotnega ekosistema. Zanimiva je razmeroma redka obrast na sedimentnem dnu (Tržaški zaliv v globini 25 m). These observations enabled the development of an optimal structure for epigrowth by sessile, filter- and suspension-feeding organisms. The configuration, size, overall rigidity, and position in relation to the bottom were among the many factors taken into consideration to prevent burial, reduce piedation, and ensure the stability of the device. The structure consists of three elements, 1) a small plastic float, 2} a weight or anchor­ing element, and 3) a flexible connection (e.g., a line) between the two. The entire, approximately fist-sized device is transported with the line either coiled around the anchoring element or bundled inside a depression in the anchoring element. When dropped into the water, it automatically unfurls and lands correctly oriented on the bottom (Figs. 6-8). The author has been granted a patent for these biological filter stations. Fig. 8: Close-up of buoyant element overgrown by 5 ascidians (Phallusia mammillata). Such densely over­grown underwater "biological filter stations" may filter over 1000 liters of water per day, SI. 8: Bližnji posnetek plavajočega elementa, ki ga je preraslo 5 bradavičastib kozolnjakov (Phallusia mam­millata). Tako na gosto poraščene podvodne "biološke precejevalne postaje" lahko precedijo več kot kubik vode na dan. The advantages of this system include: 1. not cosmetic (i.e., as in barriers preventing floating mucus from reaching beaches); it helps combat eutrophication and restore community structure 2. very srnali (not a massive or voluminous artificial reef, i.e. modular reef sets, derelict ships, etc.) 3. lightweight (no heavy equipment required for as­sembly or deployment) 4. inexpensive to produce 5. no culturing of organisms necessary (larvae settle naturally; no introduction or attraction of new or "exotic" species) 6. no maintenance, no retrieval 7. non-c.orroding, non-toxic 8. no hindrance to fisheries or navigation 9. insensitive to storms 10. invisible to tourists Most suspension- and filter-feeding organisms are known to feed virtually without interruption. The vo­lume of water filtered will vary from group to group. Typical literature values for the pumping rate of large ascidians, for example, vary between 5-17 liters of water per hour, while 2 liters/hour can be assumed for a larger bivalve (Jorgensen, 1952; Fiala-Medioni, 1979). The cal­culated volume of water pumped by an average struc­ture, based on a growth consisting of one ascidian and four bivalves, would be 400 liters/day (20 l/h x 20 h). Densely overgrown structures (figs. 8-9) could filter well over 1000 liters/day. Thus, an average structure filters j« II Fig. 9: More diverse, late-stage epigrowth on buoyant element consisting of sponges, ascidians, brittle stars, and bivalves: these epigrowth assemblages can sink to the bottom and are then indistinguishable from the ag­gregations of sessile animals that characterize this bent-hie community. Note connecting element (bottom center). SI 9: Bolj raznolika zgodnja obrast na plavajočem ele­mentu sestoji iz spužev, kozolnjakov, nitastih kačjere­pov in Školjk: takšni skupki obrasti lahko potonejo na dno, kjer jih potem ni mogoče več razločiti od skupkov sesilnih organizmov, ki so tako značilni za to bentoško skupnost. Spodaj v sredini je povezujoči element. the same amount of water that three persons discharge daily into the sewage system. The attached organisms possess a wide range of fil­tering mechanisms, resulting in an equally wide range of particle sizes being removed - from bacteria to larger flocculent material, and to a certain extent even dis­solved organic matter as well as heavy metals and other pollutants. This principle has already been put to practi­cal use in redeveloping abandoned dockyards in Eng­land: nets with cultured blue mussels (Mytilus) sus­pended in the dock basins significantly improved the quality of the water (Wilkinson et ai, 1996). This role of filter feeders may have other significant human health-related implications in eutrophicated waters, which are Michael STACHOWITSCH : BIOLOGICA L FILTER STATIONS : A NEW ARTIFICIAL REEF CONCEP T T O COMBAT... , 7-14 associated with increased toxic aigal blooms: in japan, The actual number of structures that would be em-for example, Mytilus edulis galloprovincialis has been ployed depends on a number of factors including the size shown to rapidly remove the plankton in red tide water of the basin, the magnitude of the eutrophication-induced masses (Takeda & Kurihara, 1994). Finally, in the North symptoms, and the type of filter feeders that can be Adriatic, very heavily overgrown buoyant elements of expected to settle on the buoyant elements. An appro-the biological filter stations presented here tend to sink priate number of overgrown structures would process the to the bottom, where they are virtually indistinguishable same volume of water as a waste water treatment plant ­from the aggregations that characterize the intact ben-at a fraction of the cost. This represents the only thic community. They may thus accelerate the recoloni-conceivable method of removing pelagic material from zation process of areas damaged by anoxia and other the system once nutrients or other pollutants have entered eutrophication-related effects, or help restore benthic the sea. The employment of such artificial substrates is a communities that have been depopulated by other low-tech solution to a high-tech problem. As such it chronic or acute pollution events. would have a good potential for success worldwide. BIOLOŠK E PRECEjEVALN E POSTAJE : NOV A ZAMISE L O POSTAVITV I UMETNI H MORSKI H GREBENO V KO T SREDSTV A Z A ZMANJŠEVANJ E UČINKO V EVTROFIKACIJ E V OBALNIH VODAH Michael STACHOWITSCH Instituie of Zoology, University of Vienna, A-1090 Vienna, Aithanstrasse 14 POVZETEK Plitke obalne vode so najpomembnejši morski ekosistemi tako v ekološkem kot gospodarskem pogledu. Hkrati so ti ekosistemi tudi izredno ogroženi, saj postaja vse očitneje, da je evtrofikacija zanje ena izmed največjih nevarnosti. Avtor pričujočega članka meni, da bi se morali bojevati ne le proti glavnim simptomom evtrofikacije -zmanjšani transparenfnosti vode, prekomernemu cvetenju alg, kopičenju sluzi in pomanjkanju kisika - marveč tudi proti njenim vzrokom, in sicer z izkoriščanjem naravne zmožnosti morskih organizmov precejšnja vode za odstra­njevanje snovi, lebdečih v vodi. Takšni organizmi v severnem Jadranu so kozolnjaki, školjke, spužve in črvi cevkarji, ki delujejo kot naravni zaviralci evtrofikacije. Ta sicer tehnološko zelo nezahtevna zamisel o postavitvi umetnih morskih grebenov je bila razvita in preskušena v Tržaškem zalivu in je tudi že patentirana. Zajema miniaturne "biološke precejevalne postaje", ki imajo ob majhnih stroških mnoge prednosti pred standardnimi strukturami umetnih morskih grebenov. Pravzaprav je edina strategija, ki lahko izboljša kakovost morskega okolja, potem ko se v morju nakopičijo nutrienti in drugi onesnaževalci vode. Nenazadnje pa tudi neposredno prispeva k obnavljanju ustroja in delovanja izvirne bentoške skupnosti, saj zaraslih struktur nazadnje ni mogoče več razločiti od skupkov sesilnih organizmov, ki so tako značilni za obalno morsko dno severnega Jadrana. Ključne besede: umetni morski grebeni, evtrofikacija, pomanjkanje kisika, cvetenje fitoplanktona, morski sneg, bentos, ponovna poselitev dna, severni Jadran REFERENCES Fiala-Medtoni, A., 1979. Effects of oxygen tension on pumping, filtration and oxygen uptake in the ascidian Bohnsack, j. A., D. 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