Structural changes in an aquatic microbial food web caused by inorganic nutrient addition

Abstract

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...

Scaling and power-laws in ecological systems

“…For instance, Havens (1992) demonstrated that acidification could change the parameters of freshwater plankton size spectra and Samuelsson et al (2002) show that nutrient enrichment in mesocosms resulted in higher biomass and changed plankton size structure.…”

Response of phytoplankton communities to increased anthropogenic influences (southwestern lagoon, New Caledonia)

“…This suggests that increasing contributions of 2 to 10 µm and >10 µm size classes with increasing DIN concentrations are not due to higher growth rates. A final hypothesis is advanced by Samuelsson et al (2002) that phytoplankton develops faster in enriched conditions than mesozooplankton does. A lower predation pressure on larger cells could indeed explain this dominance of larger cells in enriched conditions.…”

Phytoplankton succession and nitrogen utilization during the development of an upwelling bloom

“…Observations from both enclosure experiments (Joint et al 2002, Samuelsson et al 2002 and incubations of natural assemblages in seawater (Wetz & Wheeler 2003) have identified diatoms as major primary producers in bloom conditions. In addition to successful escape from grazing, diatom dominance is likely due to their capacity to exploit upwelling conditions far more effectively than other phytoplankton.…”