Earlier winter/spring runoff and snowmelt during warmer winters lead to lower summer chlorophyll‐a in north temperate lakes

Abstract

Winter conditions, such as ice cover and snow accumulation, are changing rapidly at northern latitudes and can have important implications for lake processes. For example, snowmelt in the watershed—a defining feature of lake hydrology because it delivers a large portion of annual nutrient inputs—is becoming earlier. Consequently, earlier and a shorter duration of snowmelt are expected to affect annual phytoplankton biomass. To test this hypothesis, we developed an index of runoff timing based on the date when 50% of cumulative runoff between January 1 and May 31 had occurred. The runoff index was computed using stream discharge for inflows, outflows, or for flows from nearby streams for 41 lakes in Europe and North America. The runoff index was then compared with summer chlorophyll‐a (Chl‐a) concentration (a proxy for phytoplankton biomass) across 5–53 years for each lake. Earlier runoff generally corresponded to lower summer Chl‐a. Furthermore, years with earlier runoff also had lower winter/spring runoff magnitude, more protracted runoff, and earlier ice‐out. We examined several lake characteristics that may regulate the strength of the relationship between runoff timing and summer Chl‐a concentrations; however, our tested covariates had little effect on the relationship. Date of ice‐out was not clearly related to summer Chl‐a concentrations. Our results indicate that ongoing changes in winter conditions may have important consequences for summer phytoplankton biomass and production.

Winter runoff events pose an unquantified continental-scale risk of high wintertime nutrient export

“…Furthermore, ecological and water quality impacts likely vary with receiving water properties, including trophic state, thermal stratification, fresh vs. salt water, and watershed:lake area ratio, and position of receiving waters within the watershed (e.g. mid-watershed vs. terminus) [2, 104,105].…”

Multi‐decadal changes in phytoplankton biomass in northern temperate lakes as seen through the prism of landscape properties

“…The relationship between precipitation and phytoplankton biomass is also known to be complex. Precipitation affects phytoplankton directly (via the direct contact of precipitation with limnetic and littoral zones of lakes) and indirectly (via run‐off and associated nutrient loading from lake catchments; Figure 3; De Senerpont Domis et al, 2013; Hrycik et al, 2021; Larsen et al, 2020; O’Neil et al, 2012; Richardson et al, 2018). For direct effects, increased precipitation is likely to enhance mixing of the water column and diluting of nutrients in lakes, which lead to lower concentrations of nutrients in water and therefore lower phytoplankton biomass (Whitehead et al, 2009).…”

“…Chl‐ a was used as a proxy for phytoplankton biomass (Boyer et al, 2009; Freeman et al, 2020; Hrycik et al, 2021). We acknowledge that Chl‐ a may not be a precise measurement of phytoplankton biomass, as the presence of chlorophyll‐ a is species specific, may vary with light and nutrient conditions (Kasprzak et al, 2008; Morel & Bricaud, 1981), or may not (Erratt et al, 2021).…”

“…and indirectly (via run-off and associated nutrient loading from lake catchments; Figure 3; De Senerpont Domis et al, 2013;Hrycik et al, 2021;Larsen et al, 2020;O'Neil et al, 2012;Richardson et al, 2018).…”

“…Therefore, to predict patterns in phytoplankton biomass in lakes, both catchments and receiving lakes should be examined for properties that influence phytoplankton biomass. However, in contrast to many studies where catchments and their lakes are explored as separate units (catchments or receiving lakes; e.g., Magnuson et al, 1990; Palmer et al, 2014; Sharma et al, 2019), there are relatively few studies where the properties of the coupled terrestrial‐aquatic system (catchments plus receiving lakes) are studied (e.g., Isles et al, 2020; Senar et al, 2018; Staehr et al, 2012) and even fewer studies where the coupled terrestrial‐aquatic systems in a large number of lakes are explored to assess phytoplankton biomass (e.g., Hrycik et al, 2021; Stomp et al, 2011).…”