Dynamics of invertebrate communities on the stony littoral of the Neva Estuary (Baltic Sea) under macroalgal blooms and bioinvasions

Abstract

1 - The Littoral zone of the Neva Estuary is strongly influenced by "macroalgal blooms" (mainly Cladophora glomerata). The present paper aims to study species composition, abundance and seasonal dynamics of littoral invertebrate communities in the Neva Estuary in order to assess their current status and the influence of macroalgal blooms on invertebrates. 2 - Littoral communities at three sites in the Neva Estuary were monitored during May to October of 2002, 2004 and 2005. Quantitative sampling of macroinvertebrates at a depth of 0.5 m was carried out with a 0.03 m2 cylindrical metal frame. Temperature, conductivity, dissolved oxygen and total phosphorus in water were measured at studied sites using a WTW Oxi330 oxygen meter, a DistWP4 conductivity meter and standard analytic techniques. 3 - A list of macroinvertebrate species includes 104 taxa in the freshwater part of the Neva Estuary and considerably fewer in its oligohaline part due to the disappearance of some species of typical freshwater groups (oligochaetes and dipterans). Amphipods Gmelinoides fasciatus and Pontogammarus robustoides, recent invaders to the Neva Estuary, were abundant species at all studied habitats of the estuary, accounting for 73% of the total biomass of zoobenthos. 4 - We obtained significant positive correlations between the density of invertebrate groups (oligochaetes, chironomids, ephemeropterans, trichopterans, amphipods) and oxygen content in water. Temporary hypoxia (0.62–2.8 mgl-1) and a 4-10-fold decrease in the density of amphipods, oligochaetes and aquatic insects were recorded in littoral habitats during decomposition of drifting filamentous algae (July-August). We conclude that intensive macroalgal blooms in the littoral zone due to the intense eutrophication of the Neva Estuary can negatively affect the density of intolerant species and groups and the structure of the invertebrate community through deterioration of oxygen conditions and increase of nutrients during intensive decomposition of drifting algae.