Temporal variation in nutrient supply by fish and watersheds at daily to decadal time scales, and its importance in sustaining primary production
There is increasing recognition that variation in resource supply strongly influences ecosystem structure and function at all levels of biological organization. In many lakes, essential terrestrially derived resources can display seasonal (e.g., snowmelt driven) and pulsed (e.g., single storm event) dynamics. A rich body of literature has developed around how these seasonal and pulsed nutrient dynamics shape ecosystem function (e.g., primary production and ecosystem respiration). However, we have begun to develop a more nuanced understanding of lake nutrient dynamics by incorporating animals as sources of nutrients. In many ecosystems, animals can transform and translocate nutrients at ecologically relevant scales. Despite this importance, we know little about the relative importance of animal-derived nutrients vs. watershed nutrient sources, and how their importance varies across multiple time scales. Understanding the patterns and dynamics of nutrient inputs is paramount to understanding and managing aquatic ecosystems, especially in the face of shifting climates and land-use patterns that will alter nutrient supply. We utilized a 19-year dataset from a eutrophic Midwestern reservoir (Acton Lake, OH, USA), which includes high frequency measurements of nutrient (P) inputs from the watershed and an ecologically dominant fish species (gizzard shad, Dorosoma cepedianum).
Watershed inputs dominated annual P loading to Acton Lake; however, we found contrasting trends at timescales more relevant to algal production. Weekly and monthly mean fish-derived P loading during the growing season (April-October) exceeded watershed inputs 56 and 47% of the time, respectively. When integrated annually over the growing season, fish-derived P excretion exceeded watershed inputs only in 3 of 19 years, but during summer (July-September), fish excretion exceeded watershed inputs in 13 of 19 years. During the summer we found that fish excretion exceeded watershed loading 78 and 72% of the time at the weekly and monthly timescales, respectively. Additionally, we found that delivery of watershed P during the growing season was 8.4, 9.2, and 6.7-fold more variable at weekly, monthly, and yearly timescales, respectively, suggesting that this source of P is less dependable than fish excretion. Our findings suggest that algal production and therefore whole lake metabolism may respond strongly to fish-derived nutrient subsidies, especially given the short generation times of algae (days). Additionally, our findings suggest eutrophication reduction efforts in Midwestern reservoirs will need to incorporate gizzard shad removal programs in addition to watershed management.