Decoupled trophic responses to long‐term recovery from acidification and associated browning in lakes

Abstract

Increases in the concentration of dissolved organic matter (DOM) have been documented in many inland waters in recent decades, a process known as “browning”. Previous studies have often used space‐for‐time substitution to examine the direct consequences of increased DOM on lake ecosystems. However, browning often occurs concomitant with other ecologically important water chemistry changes that may interact with or overwhelm any potential ecological response to browning itself. Here we examine a long‐term (~20 year) dataset of 28 lakes in the Adirondack Park, New York, USA, that have undergone strong browning in response to recovery from acidification. With these data, we explored how primary producer and zooplankton consumer populations changed during this time and what physical and chemical changes best predicted these long‐term ecosystem changes. Our results indicate that changes in primary producers are likely driven by reduced water clarity due to browning, independent of changes in nutrients, counter to previously hypothesized primary producer response to browning. In contrast, declines in calcium concomitant with browning play an important role in driving long‐term declines in zooplankton biomass. Our results indicate that responses to browning at different trophic levels are decoupled from one another. Concomitant chemical changes have important implications for our understanding of the response of aquatic ecosystems to browning.

Does browning‐induced light limitation reduce fish body growth through shifts in prey composition or reduced foraging rates?

“…Before fish addition there was an increase in chl a (proxy for pelagic phytoplankton biomass) in the BE treatment, similar to what was found in a long‐term lake study by Leach, Winslow, Hayes, and Rose (). However, higher chl a concentrations in brown waters, in the absence of increased nutrient concentrations, are probably caused by an increase in the amount of chl a per phytoplankton cell with decreased light conditions (Fennel & Boss, ; Geider, MacIntyre, & Kana, ) rather than an actual increase in phytoplankton biomass, as we see no subsequent increase in zooplankton biomass.…”

Lake browning generates a spatiotemporal mismatch between dissolved organic carbon and limiting nutrients

“…(1) we performed a multiregion spatial survey of limiting nutrients (P and N), DOC concentration, and dissolved absorbance of lakes; (2) we analyzed continental-scale spatial relationships between these endpoints using the US Environmental Protection Agency's (EPA) National Lakes Assessment (NLA) data base; and (3) we investigated long-term trends in limiting nutrients, DOC concentration, dissolved absorbance, in-lake light availability (light extinction coefficient of photosynthetically active radiation; K d ), and modeled estimates of the depth of the euphotic zone and whole-lake photosynthetic potential from a long-term data set of browning lakes in the Adirondack region of New York (Leach et al 2018b).…”

Shifting limitation of primary production: experimental support for a new model in lake ecosystems

“…The Kelly et al . (2018) model potentially reconciles the diverse relationships between DOC concentration and primary production that have been reported in the literature (Christensen et al ., 1996; Karlsson et al ., 2009; Solomon et al ., 2013; Thrane et al ., 2014; Seekell et al ., 2015; Leach et al ., 2019). Yet while the model is consistent with existing data, it has not yet been subjected to critical experimental tests.…”