Effects of gizzard shad on phytoplankton and nutrient dynamics: role of sediment feeding and fish size
2000
Schaus, Maynard H. | Vanni, Michael J.
We examined the role of the omnivorous fish, gizzard shad (Dorosoma cepedianum), in transporting nutrients from sediments into the water column and their subsequent impacts on phytoplankton. We manipulated sediment access and fish size during two mesocosm experiments to determine how these factors could alter the effects of gizzard shad on phytoplankton. In the first experiment, gizzard shad stimulated phytoplankton only when they could feed directly on lake sediments and not when benthic feeding was prevented by a mesh netting. In the second experiment, small gizzard shad stimulated phytoplankton to a greater extent than did an equivalent biomass of large gizzard shad. In the absence of gizzard shad, zooplankton became more abundant, and phytoplankton were less P limited, compared to treatments with gizzard shad present. This suggests a shift from grazer limitation in the absence of gizzard shad to direct limitation by nutrients in the presence of gizzard shad. Gizzard shad stimulated total N and P, suggesting an important nutrient transport effect. Thus, gizzard shad impact phytoplankton through an interaction of top‐down and bottom‐up effects, which are facilitated by a large detrital subsidy to this system.
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