Increased light availability and nutrient cycling by fish provide resilience against reversing eutrophication in an agriculturally impacted reservoir

Abstract

High external nutrient loads to lakes are a primary cause of eutrophication. As such, management activities in the watersheds of heavily impacted systems are commonly implemented to reduce nutrient supply to lakes with high phytoplankton biomass. Although management efforts are often successful, resilience against the reversal of eutrophication may be maintained by a variety of different drivers of phytoplankton biomass. These may include internal sources of nutrients, and increases in light availability with declining sediment loads. We used a 21‐yr dataset to examine the responses of Acton Lake, a eutrophic agriculturally impacted reservoir, to determine the relative strength of potential drivers of changes in phytoplankton biomass through time. We also identified sources of ecosystem resilience that allow the lake to remain eutrophic. Despite declining nutrient concentrations in inflow streams, chlorophyll increased in Acton Lake over approximately the first decade of our study, and remained relatively stable in the second decade. Time series modeling suggests two primary drivers of increased phytoplankton: (1) increased nutrient excretion by detritivorous fish (gizzard shad, Dorosoma cepedianum), and (2) declining sediment loads from the watershed that reduced phytoplankton light limitation. We also identify precipitation patterns and stream discharge as being possible sources of ecosystem resilience, contributing to high loads of nutrients even if concentrations in inflow streams have declined with better management. These results suggest a more comprehensive view of management efforts to reduce eutrophication may be necessary, incorporating interactions among internal nutrient sources, climate variation, and changes in light availability.

Stream Nitrogen, Phosphorus, and Sediment Concentrations Show Contrasting Long‐term Trends Associated with Agricultural Change

“…Such complex dynamics are important for downstream ecosystems. In Acton Lake, downstream of our study streams, phytoplankton biomass actually increased during the period when conservation tillage increased (Kelly et al, 2018), despite lower P concentrations in streams. During this time, interannual variation in precipitation and runoff resulted in only a subtle (and not statistically significant) decline in SRP load to the lake, despite lower concentrations in streams.…”

“…During this time, interannual variation in precipitation and runoff resulted in only a subtle (and not statistically significant) decline in SRP load to the lake, despite lower concentrations in streams. Kelly et al (2018) attributed the phytoplankton increase to decreased concentrations of inorganic SS in the lake (thereby providing more light for photosynthesis) and increased biomass of, and hence P excretion by, sediment-feeding fish, which is a significant source of P to lake phytoplankton (Vanni et al, 2006). More recently, Acton Lake phytoplankton show an increased severity of N limitation, relative to P limitation (M.J. Vanni, unpublished data, 2017), consistent with the recent declines in NO 3 and increases in SRP in streams that we show here.…”

Nitrate, ammonium, and phosphorus drive seasonal nutrient limitation of chlorophytes, cyanobacteria, and diatoms in a hyper‐eutrophic reservoir

“…Therefore, the lake receives large nutrient inputs from the terrestrial landscape (Williamson et al ; Kelly et al ). This promotes high phytoplankton biomass, with an average summer (July–September) chlorophyll, total phosphorous, and total nitrogen concentrations of 63 μ g L −1 , 94.6 μ g P L −1 , and 3.22 mg N L −1 , respectively, from 1994 to 2014 (Kelly et al ). Phytoplankton consist of a mixed assemblage of chlorophytes, cyanobacteria, diatoms, and cryptophytes with frequent dominance by cyanobacteria in summer (Dickman et al ; Hayes et al ).…”

Effectiveness of Best Management Practices to Reduce Phosphorus Loading to a Highly Eutrophic Lake

“…Both surface and bottom Chl a values frequently exceeded the 22 μg/L hypereutrophic threshold commonly used by MDEQ in its assessments of Lake Macatawa [ 9 ]. Mean Secchi disk depths indicated low transparency throughout the year, less than 1 m, suggesting eutrophic to hypereutrophic conditions [ 28 ]; high sediment loads may help keep algal blooms from becoming even more problematic due to reduced light transmission [ 29 ]. The lack of improvement in lake condition is not surprising as it often takes years, if not decades, for lake conditions to improve once the stressors are removed, and in many cases, the stressors remain in place but at reduced levels, exacerbating lake impairment [ 5 , 30 ].…”