Brenda Moraska Lafrancois

Effects of increased concentrations of inorganic nitrogen and dissolved organic matter on phytoplankton in boreal lakes with differing nutrient limitation patterns

Global changes are currently contributing to increasing concentrations of nitrogen (N) and dissolved organic matter (DOM) in boreal lakes. While these changes can affect phytoplankton, the relative importance of each of these subsidies and how their effects vary with nutrient limitation patterns remain unclear. We investigated the effects of increasing N and DOM on phytoplankton in two relatively remote boreal regions of the northern US with lakes that differ in N and DOM concentrations: Acadia National Park (ACAD) in the northeastern US, and Isle Royale National Park (ISRO) in the north-central US. Nutrient enrichment assays were conducted in two lakes from each area first to assess nutrient limitation patterns. To determine the effects of increasing N and DOM, changes in phytoplankton biomass and community structure were examined in bioassay experiments using a N gradient combined with either DOM enrichment or DOM shading treatments. Algal biomass response indicated N limitation in ISRO and N&P co-limitation in ACAD. In the N gradient experiments, algal biomass in the N-limited lake increased along the N gradient, while it was unaffected in the N&P co-limited lake. Regardless of nutrient limitation pattern, algal biomass increased with DOM enrichment, with DOM additions primarily stimulating chlorophytes, diatoms, and chrysophytes. In contrast, shading with DOM had no effect on any phytoplankton response metrics. In boreal lakes, the effects of N enrichment were dependent on nutrient limitation status of the lake, whereas the effects of DOM enrichment were observed across lakes regardless of nutrient limitation status, suggesting that increasing DOM may have widespread effects on aquatic systems.

Nitrogen deposition to lakes in national parks of the western Great Lakes region: Isotopic signatures, watershed retention, and algal shifts

Atmospheric deposition is a primary source of reactive nitrogen (Nr) to undisturbed watersheds of the Great Lakes region of the U.S., raising concerns over whether enhanced delivery over recent decades has affected lake ecosystems. The National Atmospheric Deposition Program (NADP) has been measuring Nr deposition in this region for over 35 years. Here we explore the relationships among NADP-measured Nr deposition, nitrogen stable isotopes (δ15N) in lake sediments, and the response of algal communities in 28 lakes situated in national parks of the western Great Lakes region of the U.S. We find that 36% of the lakes preserve a sediment δ15N record that is statistically correlated with some form of Nr deposition (total dissolved inorganic N, nitrate, or ammonium). Furthermore, measured long-term (since 1982) nitrogen biogeochemistry and inferred critical nitrogen loads suggest that watershed nitrogen retention and climate strongly affect whether sediment δ15N is related to Nr deposition in lake sediment records. Measurements of algal change over the last ~ 150 years suggest that Nr deposition, in-lake nutrient cycling, and watershed inputs are important factors affecting diatom community composition, in addition to direct climatic effects on lake physical limnology. The findings suggest that bulk sediment δ15N does reflect Nr deposition in some instances. In addition, this study highlights the interactive effects of Nr deposition and climate variability.

Decrease in young-of-the-year yellow perch growth rates following Bythotrephes longimanus invasion

Bythotrephes longimanus, an invasive zooplankter from Eurasia, has caused severe declines in native zooplankton communities in Rainy and Kabetogama lakes in northern Minnesota. Both lakes have experienced a 40–60% decrease in peak summer zooplankton biomass following B. longimanus establishment around 2006–2007. In these lakes, yellow perch (Perca flavescens) are a key fishery species, and young-of-the-year (YOY) yellow perch are mainly planktivorous during their first summer. This led to concern that their growth could be detrimentally affected by the depletion of zooplankton forage. We used seining data to compare growth rates of YOY yellow perch before (2001–2005) and after (2008–2012) B. longimanus establishment in Rainy and Kabetogama lakes. Nearby Lake Vermilion, assumed to have been unaffected by B. longimanus during this time period, was used as a reference for natural variation in YOY growth in the region. YOY yellow perch length was modeled as a linear function of cumulative growing degree days (GDD) throughout the summer, and the slope of the relationship was compared between pre- and post-B. longimanus time periods for the three study lakes. The two lakes with B. longimanus showed similar decreases in YOY yellow perch growth rate relative to GDD, whereas Lake Vermilion showed no evidence of a decline in growth rates during this period. The reduction in growth rates resulted in an approximate 10% decrease in mean length of YOY yellow perch at the end of the summer after B. longimanus establishment, which could lead to further effects of this invasive zooplankter at higher trophic levels.