Food Web Efficiency Research Articles

Effects of microzooplankton growth and trophic interactions on herbivory in coastal and offshore environments

We performed serial dilution experiments to estimate rates of gross phytoplankton growth (μ) and grazing mortality (m) in both eutrophic (Corpus Christi Bay, Texas, USA) and olig- otrophic (offshore Gulf of Mexico) waters. Two parallel experiments were performed in both environ- ments, with seawater pre-screened through 153 or 25 µm mesh to observe the responses of microzoo- plankton (MZP) to dilution treatments. MZP biomass changed over the duration of the experimental incubations; in several treatments, MZP net growth rates were >1 d -1 . Patterns of growth varied between dilutions and initial screening size. In the eutrophic system, the ratio of phytoplankton graz- ing mortality rate to gross phytoplankton growth rate (m/μ) was 1.10 ± 0.54 (mean ± SD) versus 0.41 ± 0.65 when screened through 153 and 25 µm mesh, respectively. This difference was attributed to cas- cading trophic interactions among MZP size groups leading to suppression of the primary herbivores in the 25 µm fraction and, in turn, a lower value of m. A food web model consisting of multiple trophic levels was constructed to examine the role of MZP growth and trophic interactions on measurements of μ and m. The model, using 3 interacting groups of MZP, was able to reproduce experimental results. Model simulations demonstrated that MZP growth during incubation leads to an overestima- tion of m. Non-linearity in the phytoplankton growth response curves was due to MZP growth and trophic interactions in these model simulations, as variable feeding responses were not incorporated into the models. Trophic interactions among MZP can provide context to measurements of μ and m and insight into microbial food web efficiency.

Efficiency of a phytoplankton‐based and a bacterial‐based food web in a pelagic marine system

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.

Prey fish exploitation, salmonine production, and pelagic food web efficiency in Lake Ontario

Estimates of production and predation rates from bioenergetic models of chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), and lake trout (Salvelinus namaycush) suggest a long-term decline in their gross conversion efficiency (gross production/prey consumption) and the gross production to biomass ratio in Lake Ontario during 1978-1994. The former pattern was caused primarily by a declining trend in adult alewife (Alosa pseudoharengus) energy density during 1978-1985; the latter pattern resulted from reductions in growth rates (coho salmon) and a buildup of the older age-classes in the population (lake trout) over time. Model results suggest that over 100 and 25% of the annual production of adult alewife and rainbow smelt (Osmerus mordax), respectively, was consumed by salmonines during 1990 in Lake Ontario; hence, we claim that recent observations of reduced salmonine growth in Lake Ontario may be a result of prey limitation. Energy transfer from primary production to salmonines appeared to be more efficient in Lake Ontario than in Lake Michigan, probably due to higher stocking levels per unit area and higher densities of preferred prey fish in Lake Ontario. Through separate analyses, we arrived at conflicting conclusions concerning the sustainability of the food web configuration in Lake Ontario during 1990.

Prey fish exploitation, salmonine production, and pelagic food web efficiency in Lake Ontario

Prey fish exploitation, salmonine production, and pelagic food web efficiency in Lake Ontario

Toward the development of generally applicable models of the microbial loop in aquatic ecosystems

Simulation modeling has been an integral, albeit ad hoc, component of the field of aquatic microbial ecology for the past two decades. One of the most critical steps in simulation modeling is the initial formulation of a clear set of questions and goals. It is doubtful that a single generic model could be constructed to address adequately all questions of interest concerning the microbial loop because of the tremendous range in time scales that define these questions. Progress in the field of aquatic microbial ecology will benefit from an integrated research program including experimental and modeling approaches. A submodel of bacterial utilization of various qualities of organic matter that we have under construction is presented. This submodel will be a component of a larger model to evaluate the effects of quality and quantity of organic matter and inorganic nutrient inputs on estuarine food web structure and efficiency. The overall model will be general enough in its structure that it should be applicable to a wide range of questions concerning the microbial loop, with time scales ranging from hours to days.

Efficiencies of a tropical freshwater pond community

Individual and population net efficiencies of trophic levels of tropical freshwater pond community increased with increasing trophic levels. The individual and population net efficiencies of this study are higher compared to temperate forms. The assimilation and ecological efficiencies are calculated for different trophic levels and presented. A new term foodweb efficiency is coined, defined and calculated for the first time and presented. The significance of foodweb efficiencies is also discussed.