The food web efficiency in two contrasting food webs, one phytoplankton based and one bacteria based, was studied in a mesocosm experiment using seawater from the northern Baltic Sea. Organisms included in the experiment were bacteria, phytoplankton, protozoa, and mesozooplankton (copepods). A phytoplankton-based food web was generated by incubating at a high light level with the addition of nitrogen and phosphorus (NP). A bacteria-based food web was created by adding carbon, nitrogen, and phosphorus (CNP) and incubating at a lower light level. In the CNP treatment bacteria dominated the productivity (91%), while in the NP treatment phytoplankton were dominant producers (74%). The phytoplankton community in the NP treatment was dominated by autotrophic nanoflagellates. The food web efficiency, defined as mesozooplankton productivity per basal productivity (phytoplankton + bacteria), was 22% in the phytoplankton-based food web and 2% in the bacteria-based food web. This discrepancy could be explained by 1-2 extra trophic levels in the bacteria-based food web where carbon passed through flagellates and ciliates before reaching mesozooplankton, while in the phytoplankton-based food web there was a direct pathway from phytoplankton to mesozooplankton. The results were supported by stable isotope analysis of mesozooplankton. We propose that climate change, with increased precipitation and river runoff in the Baltic Sea, might favor a bacteria-based food web and thereby reduce pelagic productivity at higher trophic levels.

The effect of temperature changes on the marine pelagic food web was studied in three successive mesocosm experiments, performed during the spring bloom 2001 in the northern Baltic Sea. The temperature was varied from 5 to 20°C in each experiment, running over a 3-week period. The experiments included food webs of at least four trophic levels: (1) phytoplankton-bacteria, (2) flagellates, (3) ciliates and (4) metazooplankton. The results showed that heterotrophic to autotrophic biomass ratio (H/A) increased 5 times when temperature was raised from 5 to 10°C. In agreement, the carbon fixation to respiration ratio indicated a decrease of six times over the same temperature range. Furthermore, the sedimentation decreased by 45% when the temperature was elevated from 5 to 10°C, probably as a consequence of the increased respiration losses and bacterial biodegradation of settling material. Analyzed parameters, thus, indicated that the degree of heterotrophy increased in the temperature interval of 5-10°C. Above 10°C, the analyzed parameters in general were more stable. Our results indicate that moderately elevated seawater temperatures, due to climate change or weather alterations, may affect the entire ecosystem function in temperate sea areas by altering the balance between autotrophy and heterotrophy.

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Present theories regarding nutrient status of aquatic systems and plankton size structure, food web length and nutritional modes were tested in a microcosm experiment using water from the northern Baltic Sea. The different trophic levels included were those of pico-, nano-and microplankton, representing bacteria, flagellates, ciliates and diatoms. Nutrient enrichment resulted in a higher biomass and changed size-structure of the organisms. The nutrient-enriched microcosms changed from a picoplankton-dominated system to a microplankton-dominated system. In contrast, the nutrient-poor treatment remained at the pico-and nanoplankton levels. The increase in biomass within the enriched treatment resulted in a lengthening of the food chain to include a firmly established ciliate community. In accordance to this, no shift to inedible or toxic species was observed. In the nutrient-poor treatment the ciliates were kept at very low population densities. Furthermore, a potentially mixotrophic flagellate (Chrysochromulina sp.) was able to maintain a positive population growth in the nutrient-poor treatment, while specialised heterotrophic or autotrophic flagellates (Paraphysomonas imperforata Lucas and Plagioselmis prolonga Butcher) decreased in numbers. In the nutrient enriched system the potential mixotrophic flagellate coexisted with specialised heterotrophic and autotrophic forms. A clear succession pattern of diatoms was observed, illustrating early and late successional species within diatom populations.KEY WORDS: Bacteria · Flagellates · Ciliates · Phytoplankton · Mixotrophy · Food web length · Nutrient status Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 29: [29][30][31][32][33][34][35][36][37][38] 2002 Another factor that might hinder a lengthening of the food chain with increasing nutrient loading is that predation-resistant species can be favoured as a response to strong predation pressure (Jürgens & Montserrat Sala 2000). This can result in resource-controlled population dynamics (Vanni 1987, Balciunas & Lawler 1995, Brett & Goldman 1997.The food webs in natural systems are more complicated than simple theoretical models, which only include autotrophic and heterotrophic species. Mixotrophy among protists of different taxonomic groups has been shown to be common in marine, brackish and freshwater environments (e.g. Sanders 1991). An advantage of being a mixotroph compared to a specialist autotroph or heterotroph is assumed to be better survival during periods of resource limitation (e.g. Andersson et al. 1989, Nygaard & Tobiesen 1993. During these situations, mixotrophs are superior to autotrophs due to their ability to retrieve phosphorus and nitrogen through phagotrophy, and superior to heterotrophs due to their photosynthetic capacity (Nygaard & Tobiesen 1993). Although this theory is well established, there are only a few competition experiments published on heterotrophic, mixotrophic and autotrophic flagellates. These studies include a mixotroph...