A shady phytoplankton paradox: when phytoplankton increases under low light

Abstract

Light is a fundamental driver of ecosystem dynamics, affecting the rate of photosynthesis and primary production. In spite of its importance, less is known about its community-scale effects on aquatic ecosystems compared with those of nutrient loading. Understanding light limitation is also important for ecosystem management, as human activities have been rapidly altering light availability to aquatic ecosystems. Here we show that decreasing light can paradoxically increase phytoplankton abundance in shallow lakes. Our results, based on field manipulation experiments, field observations and models, suggest that, under competition for light and nutrients between phytoplankton and submersed macrophytes, alternative stable states are possible under high-light supply. In a macrophyte-dominated state, as light decreases phytoplankton density increases, because macrophytes (which effectively compete for nutrients released from the sediment) are more severely affected by light reduction. Our results demonstrate how species interactions with spatial heterogeneity can cause an unexpected outcome in complex ecosystems. An implication of our findings is that partial surface shading for controlling harmful algal bloom may, counterintuitively, increase phytoplankton abundance by decreasing macrophytes. Therefore, to predict how shallow lake ecosystems respond to environmental perturbations, it is essential to consider effects of light on the interactions between pelagic and benthic producers.

Submerged macrophytes affect the temporal variability of aquatic ecosystems

“…In shallow lakes, the positive feedback between light transmission and macrophyte biomass is an important reason why macrophytes help maintain a clear water state over a wide range of nutrient loading (Blindow et al,2006; Blindow et al, 1998; Scheffer et al, 1993, 2003). Many types of macrophytes are efficient at taking up nutrients from the water and, if rooted, from the sediment, which can limit phytoplankton growth at low to intermediate nutrient loading (Yamamichi et al, 2018). Furthermore, macrophytes can reduce fish predation pressure on the zooplankton communities that graze on phytoplankton (Jeppesen et al., 1997), and can also produce allelopathic chemicals that inhibit phytoplankton growth (Gross, 2003; Hilt & Gross, 2008; Nakai et al, 2012).…”

Changes in Heterotrophic Picoplankton Community Structure after Induction of a Phytoplankton Bloom under Different Light Regimes

“…Light as a driver of ecosystem dynamics has been explored in a number of studies, but has generally received less attention compared to other environmental factors like temperature and nutrients. It has been shown, however, that light may have direct or indirect effects at cellular to community levels [17][18][19][20]. For instance, stimulation of bacterial clades containing aerobic anoxygenic phototrophs (Rhodobacteraceae and NOR5) and inhibition of SAR11 [21][22][23] have been shown.…”

A unified framework for herbivore-to-producer biomass ratio reveals the relative influence of four ecological factors

“…To initiate the experiment, we equally divided each of the two ponds into four sections using vinyl-coated canvas curtains, and randomly assigned the four sections to either high-shade (64% shading), mid-shade (47% shading), low-shade (33% shading), or no-shade treatments (no shading). Shading in each treatment was made using opaque floating mats (6 m diameter; Solar-cell SunBlanket, Century Products, Inc., Georgia, USA) 31 . The floating mats were deployed silvered side up to reflect sunlight and blue side down to avoid pond heating.…”

“…1). At the Neimi Road Site (Unit 2) of CUEPF, 50 ponds were built in 1964 and have since been used for various field experiments in aquatic ecology 31,33,40 . Each pond has a 0.09 ha surface area (30 × 30 m) and is 1.5 m deep.…”

“…In each pond, we randomly assigned the four sections to one of four treatments: high-shade (64% shading), mid-shade (47% shading), low-shade (33% shading), or no-shade treatments (no shading). Shading in each treatment was made using different number of opaque floating mats (6 m diameter; Solar-cell SunBlanket, Century Products, Inc., Georgia, USA) 31 (Fig. 1).…”