Impact of an omnivorous fish on nutrient dynamics in mesocosms via excretion and bioturbation

Background/Question/Methods

Gizzard shad (Dorosoma cepedianum) can have a major influence on nutrient cycling in eutrophic and hypereutrophic lakes, where it can be the dominant fish species.  Gizzard shad can release nutrients from sediments into the water column via excretion and bioturbation, potentially increasing phytoplankton biomass and decreasing water clarity.  Biomanipulation, or the removal of benthic feeding fishes, has been used by lake managers as a tool to improve water quality in many lakes in Europe and North America.  The St. Johns River Water Management District has undertaken a large-scale removal of gizzard shad from Lake Apopka, FL (over 6.4 million kilograms of gizzard shad removed during 1993-2007), as a means to reduce lake phosphorus and phytoplankton concentrations, and improve water clarity.  To better understand the impact of this massive fish removal on lake nutrient cycles, we conducted tank microcosm experiments during summer 2006 and 2007 that manipulated the density of fish, sediment access, and the presence of phytoplankton.  To examine the importance of excretion and bioturbation mechanisms, we compared the responses across experiments and manually stirred the sediments in some of the tanks during the March 2006 experiment.

Results/Conclusions

Gizzard shad typically only stimulated phytoplankton biomass and total and particulate nutrient concentrations when fish could feed directly on sediments.  In most cases, there was an increased proportion of nutrients (both N and P) in particulate form, indicating that gizzard shad can have some impact on turbidity of the water column via bioturbation, in addition to excretion effects.  Increases in turbidity, total and particulate nutrients, were highest with high fish biomass, as was expected.  Tanks where sediments were manually disturbed with a paddle had a modest increase of total dissolved P concentrations, even though turbidity and chlorophyll were substantially elevated over control tanks.  In this system, gizzard shad appear to be important in the excretion of nutrients, the resuspension of phytoplankton, and to some extent the release of dissolved P directly from sediment via bioturbation.  In other systems where benthivores such as common carp (Cyprinus carpio) extensively disturb sediments, there may be a greater impact of bioturbation, especially if pore water nutrients remain mobilized in the water column or if sediment particles limit light availability to aquatic plants.