Dynamics of primary productivity in relation to submerged vegetation of a shallow, eutrophic lagoon: a field and mesocosm study

Abstract

Aquatic ecosystems nowadays are under constant pressure, either from recent or historical events. In most systems with increased nutrient supply, submerged macrophytes got replaced by another stable state, dominated by phytoplankton as main primary producer. Yet, reducing the nutrient supply did not yield the aimed goal of restored habitats for submerged macrophytes in systems worldwide. The question arises, why submerged macrophytes do not re-colonize, and if they are actually competitive. Therefore, primary production assays were conducted in ex-situ bentho-pelagic mesocosms and compared to the actual ecosystem, a turbid brackish lagoon of the southern Baltic Sea. Mesocosm were either manipulated to be colonized by macrophytes, or stayed phytoplankton dominated. Oxygen evolution was monitored over a period of five months in 5 min (mesocosms) to 10 min (ecosystem) intervals. Surface and depth-integrated production was calculated to analyse seasonal and areal resolved production patterns. It was found that macrophyte mesocosms were more stable, when considering only surface O 2 production. However, calculating depth-integrated production resulted in net-heterotrophy in both shallow mesocosms approaches and the actual ecosystem. This heterotrophy is likely mediated by sediment respiration and POC accumulation in mesocosms, and a low share of productive to respiring water column in the actual ecosystem. Therefore, it seems unlikely that macrophytes will re-settle, as constant net-heterotrophy may allow for high-nutrient turnover at sediment-water interfaces and within the water column, favouring phytoplankton. Changes within the ecosystem cannot be expected without further restoration measures within and in the systems proximity.

Increases in temperature and freshwater inputs will shift grazing patterns of a coastal mesograzer on foundation species

“…These broader temperature acclimation abilities may help to permanently introduce G. tigrinus within the Baltic Sea, as sea surface temperature is already higher there than in other ocean parts (Reusch et al, 2018). Furthermore, the endurance of hypoxic conditions is an advantage in eutrophic coastal waters of the Baltic, as redox conditions within the reed belt and adjacent macrophyte stands can change fast (Berthold & Paar, 2021; Karstens et al, 2015), and an increase in nutrients is challenging for submerged macrophytes in the coastal water bodies of the southern Baltic Sea (Paar et al, 2021), stressing habitats of native gammarids. On the contrary, an in silico study assumed that G. tigrinus has actually a narrower niche than native gammarids in the northern Baltic Sea (Herkül et al, 2016).…”

Mesopredator-mediated trophic cascade can break persistent phytoplankton blooms in coastal waters

“…This nutrient increase favored phytoplankton directly, as seen by increasing Chl a (Figure 3), as well as probably epiphytic biomass (Xie et al 2013). Ultimately, this increasing phytoplankton biomass will result in lower primary production of all compartments (phytoplankton, macrophytes, epiphytes) as seen in lakes (Blindow et al, 2006) and mesocosms (Berthold and Paar 2021).…”

“…Ultimately, this increasing phytoplankton biomass will cause high light attenuation which lowers the net-productive part of the water column (Duarte et al 2002). This increased light attenuation thus results in lower primary production of all compartments (phytoplankton, macrophytes, epiphytes) as seen in lakes (Blindow et al, 2006) and mesocosms (Berthold and Paar 2021).…”