Lost connectivity between a coastal lagoon and the sea – implications of floodgate closure for benthic macroinvertebrates

Abstract

Hydrological connectivity between coastal lagoons and marine water plays a major role in ecosystem health. Natural brackish habitats show an intrinsic spatial dispersion of benthic fauna when free water exchange is guaranteed. Habitat alteration caused by frequent and long-lasting floodgate closing is one of the major reasons for the decline in biological diversity and increased eutrophication of water. This type of disturbance, being a substantial stress (e.g. change in salinity, nutrient concentrations, and water cycle) for benthic macroinvertebrates, is presented on the example of the Jamno Lagoon (coastal lake), which is located in the middle part of the southern coast of the Baltic Sea (54°17′N/16°08′E). Floodgate closing is not itself a stressor for macrofauna, but if it occurs, it can be recognized as a specific trigger for the changes inhabitat conditions. An assessment of changes in benthic fauna species richness, community composition, density and biomasson the background of modified habitat conditions was performed between two different stages: (1) free water exchange (FF) and (2) separation from the sea (NF). Disturbance in the sea water inflow induced profound changes in the macrozoobenthic structure, decreasing its biomass by 60% and its density by 50%. Only the Shannon α-diversity index (H′) did not show significant changes between the stages. The most significantdecreaseswere in the spatial distribution of Oligochaeta and Chironomus f.l. plumosus L. (Diptera) descriptors. The results showed significant differences in the physiochemical parameters of water between the FF and NF phases. Soon after the inflow of seawater was blocked, a distinct decrease in Cl− and Na+ concentrations was accompanied by an increase in total dissolved solids (TDS), Chl-a and NH4+. Apart from influence of seawater (concentration of Na+ and Cl− ions), the qualitative and quantitative structure of macrobenthic invertebrates was also driven by Chl-a, pH, and salinity. The obtained results justify the role of benthic fauna as a good indicator of anthropogenic disturbance and a plausible component of a decision support system in the proper management of coastal aquatic ecosystems.