Coastal darkening exacerbates eutrophication symptoms through bottom‐up and top‐down control modification

Abstract

Coastal eutrophication due to excessive anthropogenic nutrient loading is a major threat worldwide, and especially in estuaries and semi-enclosed waterbodies, like the brackish Baltic Sea. In addition, coastal waters may become darker (coastal darkening) due to increased input of colored compounds from terrestrial run-off and sediment resuspension. Still, the effects of darkening on coastal food web responses to eutrophication are unknown. In a mesocosm experiment with benthic and pelagic habitats, we manipulated nutrient loading, presence of fish and light availability to disentangle bottom-up and top-down control of eutrophication symptoms in ambient and darkened waters. Overall, we found higher pelagic Chlorophyll a concentrations (a proxy of algal biomass) with darkening and with nutrient enrichment in both clear and dark waters. Albeit fish had a strong impact on zooplankton and zoobenthos, they had no cascading effect on algae. We conclude that coastal darkening due to changes in land use and climate change can pose an additional challenge concerning the recovery of coastal waters from eutrophication.

Brownification shapes the food web of aquatic invertebrates: a review

“…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 presence or absence of fish tends to diverge the effects brownification on aquatic invertebrates. Among macroinvertebrates, brownification negatively affected chironomid biomass with fish presence, but not in the absence of fish (Garnier et al, 2023). With fish presence, chaoborid larvae were found to use habitats with low oxygen levels to avoid fish in waters with high DOC concentrations (Horppila et al, 2018).…”

“…Other European countries contributed to 12 papers, while South America contributed to 2 papers and China contributed 1 paper (Table 1, Figure 2). Freshwater ecosystems, especially boreal lakes, received most attention (Table 1), while only four papers investigated seawater in the field (Bandara et al, 2022;Soulié et al, 2022) and the mesocosms (Garnier et al, 2023). Zooplankton has received more attention than macroinvertebrates in this topic.…”

“…Marine habitats. Four papers studied aquatic invertebrates in browning marine habitats (Bandara et al, 2022;Soulié et al, 2022;Garnier et al, 2023) in the context of eutrophication.…”

“…Eutrophication is known to enhance primary production in the sea (Andersen et al, 2017) and alter the food web via bottom-up effects, as well as top-down control (Soulié et al, 2022;Garnier et al, 2023); its symptoms can be amplified in browning water (Garnier et al, 2023).…”