Carbon flux rates in shallow lakes: A comparison of detritivorous fish, lake metabolism, and sediment sequestration

Background/Question/Methods

Shallow lakes have the potential to sequester large amounts of organic carbon in their sediment and play a disproportionately large role in the global carbon cycle. However, the sequestration rates in these lakes are highly variable. The consumption of detritus by detritivorous fish and aquatic invertebrates may reduce sequestration rates and contribute to this variability. The fathead minnow (Pimephales promelas) is a detritivorous fish common in prairie shallow lakes in North America and Canada, and can reach high densities (107 fish m²) and biomass (48 g m^-2) in these systems. We assessed fathead minnow population dynamics and prey consumption in three shallow Minnesota prairie lakes during summers of 2001 and 2002. We used bioenergetics models to estimate fathead minnow consumption and respiration of organic carbon from lake detritus. We then compared the consumption and respiration rates from the fish to estimates of organic carbon sequestration rates in sediments via sediment cores from similar lakes.  We also compared carbon flux rates of fish to whole-lake estimates of gross primary production and respiration estimated via continuously deployed sondes measuring changes in temperature and dissolved oxygen.  

Results/Conclusions

Results showed the three fish populations on average consumed 0.6 g m^-2 d^-1 of detritus (dry weight) from May through August of both 2001 and 2002. This represents 0.3 g m^-2 d^-1 organic carbon consumed from lake detritus, with 0.1 g m^-2 d^-1 of the organic carbon converted to CO₂ via respiration of the fish. Larval fathead minnows played a disproportionately large role consuming on average 69% of the total detritus consumed in each lake. Estimates of organic carbon sequestration in sediments via sediment cores averaged 0.2 g m^-2 d^-1 (range 0.1 – 0.4), slightly lower than consumption rates of sediments by fish. Moreover, estimates of total fish carbon consumption (detritus + invertebrates) indicated minnows consumed 11% of lake gross primary production during the summer. The similarity in fish consumption and respiration rates of carbon compared to lake metabolism and sediment sequestration rates indicates fathead minnows are an important component of the carbon cycle in these ecosystems. Minnow abundance and distribution is highly variable among prairie lakes, with some lakes supporting high densities while others are fishless. This variability likely translates into different carbon dynamics in lakes with fish compared to fishless sites.