Pelagic food web configurations at different levels of nutrient richness and their implications for the ratio fish production:primary production

Abstract

Based on existing knowledge about phytoplankton responses to nutrients and food size spectra of herbivorous zooplankton, three different configurations of pelagic food webs are proposed for three different types of marine nutrient regimes: (1) upwelling systems, (2) oligotrophic oceanic systems, (3) eutrophicated coastal systems. Upwelling systems are characterised by high levels of plant nutrients and high ratios of Si to Nand P. Phytoplankton consists mainly of diatoms together with a sub dominant contribution of flagellates. Most phytoplankton falls into the food spectrum of herbivorous, crustacean zooplankton. Therefore, herbivorous crustaceans occupy trophic level 2 and zooplanktivorous fish occupy trophic level 3. Phytoplankton in oligotrophic, oceanic systems is dominated by picoplankton, which are too small to be ingested by copepods. Most primary production is channelled through the 'microbial loop' (picoplankton -heterotrophic nanoflagellates -ciliates). Sporadically, pelagic tunicates also consume a substantial proportion of primary production. Herbivorous crustaceans feed on heterotrophic nanoflagellates and ciliates, thus occupying a food chain position between 3 and 4, which leads to a food chain position between 4 and 5 for zooplanktivorous fish. By cultural eutrophication, Nand P availability are elevated while Si remains unaffected or even declines. Diatoms decrease in relative importance while summer blooms of inedible algae (Phaeocystis, toxic dinoflagellates, toxic prymnesiophyceae, etc.) prevail. The spring bloom may still contain a substantial contribution of diatoms. The production of the inedible algae enters the pelagic energy flow via the detritus food chain: DOC release by cell lysis -bacteria -heterotrophic nanoflagellates -ciliates. Accordingly, crustacean zooplankton occupy food chain position 4 to 5 during the non-diatom seasons. Ecological efficiency considerations lead to the conclusion that fish production:primary production ratios should be highest in upwelling systems and substantially lower in oligotrophic and in culturally eutrophicated systems. Further losses of fish production may occur when carnivorous, gelatinous zooplankton Gellyfish) replace fish.

Has eutrophication promoted forage fish production in the Baltic Sea?

“…A clearly demonstrated biological effect of the increased nutrients was the 3-to 5-fold increase in macrobenthic biomass between 1920/1923 and 1976/1977 in shallower waters not impacted by anoxia (Cederwall and Elmgren 1980). We recognize that nutrients are not directly influencing secondary production but via food web interactions at lower tropic levels (Sommer et al 2002). Thus, ideally, the Table 2 The variables explaining significant (*p \ 0.05) amount of variability in sprat and herring recruitment in southwestern (SW), southeastern (SE) and northern (N) areas of the Baltic Proper (spawning stock biomass (SSB), seas surface temperature (SST), Baltic Sea environmental index (BSE)).…”

Effects of low dissolved oxygen on planula settlement, polyp growth and asexual reproduction of Aurelia aurita

“…In order to predict Effects of low dissolved oxygen on planulae and polyps of Aurelia aurita the overall effect of hypoxia on the jellyfish population, more information is needed on the habitat requirements and environmental conditions necessary to sustain polyp colonies of A. aurita. These will also help to elucidate how jellyfish could become dominant over fishes in regions affected by severe hypoxia, and how energy pathways are altered in these ecosystems (Breitburg et al 1999, Sommer et al 2002.…”

Effects of the Operating Nuclear Power Plant on Marine Ecology and Environment - A Case Study of Daya Bay in China

“…Although the mean annual abundances of phytoplankton show a slight decrease trend from 1999 to 2004, species and values of the phytoplankton of Daya Bay were increasing that might be due to high ratios of TIN to P and Si to P occurring in recent years (Sommer et al, 2002). Annual mean values of chlorophyll a were 1.83-3.78 mg m -3 in different seasons from 1985 to 2004, the higher values were always found in autumn and summer.…”

“…Although main species of the zooplankton in Daya Bay had a decreasing trend from 46 of 60 familiar species in 1983 to 36 of 60 familiar species in 2004 (Fig.14), the annual mean individual 1991-1992 1992-1993 1994-1995 1996-1997 Year (Table 3). One reason might be the strictly enforced regulations relating to the marine environment and fisheries from June to August in each year since 1995, and another reason might be high levels of plant nutrients and high ratios of Si to N and P, most phytoplankton falls into the food spectrum of herbivorous, crustacean zooplankton in recent years (Sommer et al, 2002(Sommer et al, , 2008. (Table 4).…”