Changes in mixing depth reduce phytoplankton biomass in an Arctic lake: Results from a whole-lake experiment

Abstract

Research has revealed contradictory responses of primary producers in Arctic lakes to increasing temperatures, making it unclear how future warming and climate change will alter lake productivity. We conducted a whole-lake manipulation to examine the effect of altered thermal structure on phytoplankton biomass in a lake in Greenland, one of the most rapidly warming regions of the world. Deepened lake mixing (from 4 to 8 m) in the experimental lake led to significant declines in phytoplankton biomass in the sediment traps despite warmer surface waters, indicating that changes in mixing depth may negate a positive, direct effect of warming on primary producer biomass. Light limitation (induced by deeper mixing) of phytoplankton played a greater role than temperature in structuring these Arctic lake phytoplankton communities. To put the manipulation in context, we surveyed twenty-four lakes across western Greenland to determine the strongest regional predictors of phytoplankton density. Across the landscape, lake chemistry and light attenuation in the epilimnion were the most important predictors of algal assemblages in the survey lakes. Though temperature can directly influence lake ecosystems, multiple factors will affect mixing depths of Arctic lakes, potentially leading to variable effects of warming on phytoplankton biomass and community structure.

Ecological impacts of photosynthetic light harvesting in changing aquatic environments: A systematic literature map

“…When being dragged through different light regimes in interaction with variable nutrient supply dependent on the mixing depth, phytoplankton communities are expected to change in their species composition (Marzetz et al, 2020 ; Saros et al, 2012 ). Thus, increased vertical mixing depth acts selectively by limiting light in spectrum and intensity at depth and reduces phytoplankton growth due to longer periods at depth (Lehman, 2002 ; Northington et al, 2019 ). On the other hand, when the mixing depth or the water column itself is shallow enough for light being not limited, the higher availability of nutrients from hypolimnic water and sediments would promote phytoplankton growth (Planas & Paquet, 2016 ).…”

Under-ice limnology of coastal valley lakes at the edge of the Arctic Ocean

“…Higher nutrient inputs and/or longer ice-free seasons are predicted for polar regions; both imply increases in phytoplankton production. Changes in the ice cover and mixing regimes of polar lakes have been observed to lead to ecological reorganizations towards either greater benthic or planktonic production, depending on local limnological factors (Rautio et al 2011, Northington et al 2019. Nutrient concentrations were especially high in Bottom Lake, and given the dominance of ice and snow cover throughout most of the year, the lake could be approaching a trophic threshold between benthic versus planktonic dominance of primary productivity (Karlsson et al 2009, Vadeboncoeur et al 2003.…”

Element cycling and aquatic function in a changing Arctic

“…Where warming additionally leads to increased delivery of chromophoric dissolved organic matter to aquatic systems, stronger near‐surface absorption of thermal energy from light may further strengthen stratification and lead to shallower mixed layers (Wauthy and Rautio 2020 b ). Where stratification is already established, however, longer ice‐free seasons may also enable mixed layers to deepen (Northington et al 2019). These changes in stratification, while variable depending on the present stratification status of the lake (as governed by latitude, lake depth, and transparency; Lewis 1983; Woolway and Merchant 2019), can lead to pronounced changes in biogeochemistry and ecology.…”

“…For example, longer duration of stratification in Greenland has been associated with a longer period of hypolimnetic anoxia and greater water column inventories of CH 4 (Cadieux et al 2017), while strengthened stratification in northern Canadian thaw ponds has been shown to affect zooplankton distribution (Wauthy and Rautio 2020 a ). In contrast, when longer ice‐free seasons cause thermoclines to deepen, reduced water column primary production can result (Northington et al 2019). Perhaps the most striking example of the effects of changing stratification regimes comes from High Arctic lakes that transition from being perennially ice‐covered, to briefly losing ice cover during the summer months (e.g., Veillette et al 2010).…”