Herbivorous cladoceran essential fatty acid and cholesterol content across a phosphorous and DOC gradients of boreal lakes—Importance of diet selection

Abstract

1. Eutrophication has been shown to increase production of nutritionally low-quality cyanobacteria and decrease the long-chain polyunsaturated fatty acid (PUFA) content of seston. Contrarily, lake browning inhibits cyanobacteria contribution in seston and favours poorly grazable mixotrophic algal species. These environmental changes have probable impacts on the diet and long-chain PUFA content of primary consumers. However, herbivorous zooplankton may preferentially retain PUFAs through diet selection for optimal growth and reproduction, but such selective feeding is challenging to document in nature owing to the difficulties in quantifying zooplankton diet.2. Here, we sampled seston and herbivorous cladocerans (Daphnia sp. and Bosmina sp.) from lakes (n = 23) in Finland along eutrophication (total phosphorous) and browning gradients (dissolved organic carbon [DOC]). We analysed the fatty acid content of seston (mg FA/g POC [particular organic content]) and cladocerans (mg FA/g C), and estimated available and consumed diet biomass percentages with quantitative fatty acid signature analysis. Cladoceran diet preference was evaluated as the difference between consumed and available food sources, to understand if they preferentially retain high nutritional quality diet.3. Generally, lake chemistry and morphometry poorly explained seston and cladoceran long-chain PUFA contents. However, multiple linear models for shorter chain PUFAs (linoleic acid [LA] and alpha-linolenic acid [ALA]) performed better in explaining variation in the LA and ALA content of seston (20% and 11%) and cladocerans (36% and 46%, respectively). The factors most strongly and positively associated with the LA and ALA content of seston and cladocerans were phosphorus and DOC concentrations, respectively. 4. Seston and cladoceran PUFA contents were clearly uncorrelated. In most of the sampled lakes, high-quality diet (i.e., diatoms and cryptomonads) was preferred by cladocerans and low-quality diet (cyanobacteria) was avoided. Lake chemistry poorly explained cladoceran diet preference, but high-quality preference was positively associated with lake average depth.

Brownification shapes the food web of aquatic invertebrates: a review

“…More specifically, the effects were positive on rotifer and copepod biomass (Garnier et al, 2023;Leech et al, 2018), but not on chironomid biomass (Garnier et al, 2023) nor cladoceran biomass (Leech et al, 2018;Keva et al, 2023). Brownification or eutrophication had no negative effects on the transfer of essential PUFAs from phytoplankton to herbivorous cladocerans (Keva et al, 2023). These results suggest brownification may exaggerate eutrophication symptoms in certain taxa and modify food webs.…”

“…Brownification plus eutrophication had stronger positive effects on the total zooplankton biomass than brownification alone. More specifically, the effects were positive on rotifer and copepod biomass (Garnier et al, 2023;Leech et al, 2018), but not on chironomid biomass (Garnier et al, 2023) nor cladoceran biomass (Leech et al, 2018;Keva et al, 2023). Brownification or eutrophication had no negative effects on the transfer of essential PUFAs from phytoplankton to herbivorous cladocerans (Keva et al, 2023).…”

“…The increase of toxic Microcystis was negatively associated with Daphnia biomass (Urrutia-Cordero et al, 2016). Yet, the pattern was not universal: in another study, the browning-induced total cyanobacteria biomass increase was positively associated with the biomass of zooplankton, such as daphnids and copepods (MacKeigan et al, 2023), which may have partially resulted from zooplankton choosing high-quality algal groups for food (Keva et al, 2023). The findings may have resulted from improved fatty acid quality via cyanobacteria being ingested by heterotrophic nanoflagellates, increasing food quality for zooplankton (Bec et al, 2006).…”