How much can the occurrence and coverage of charophytes in an estuarine lagoon (Curonian Lagoon) be explained by environmental factors?

Highlights

• Biotic and abiotic factors influence the distribution of charophytes in the lagoon.

• Salinity and wave exposure were the most important explanatory factors.

• Occurrence of brackish and freshwater species were restricted by salinity of 0.4.

• Chara contraria and Tolypella nidifica were positively related to wave exposure.

• Competitive interactions due to stands of pondweeds limited Nitellopsis obtusa cover.

Abstract

Aquatic macrophytes play an important role in the structure and functioning of lagoon ecosystems. We assessed the importance of biotic and abiotic environmental factors in controlling spatial patterns of charophyte occurrence and coverage in the Curonian Lagoon. The relationships among five charophyte species, abiotic and biotic environmental factors were assessed using canonical correspondence analysis and multivariate regression trees. Salinity and wave exposure were the most important environmental factors explaining the occurrence and coverage of charophytes species in the estuarine part of the lagoon. The salinity gradient restricted the occurrence of brackish water species (especially Chara baltica and Tolypella nidifica) and freshwater species (especially Nitellopsis obtusa). The occurrence and coverage of C. contraria and T. nidifica were positively associated with the wave exposure and related factors (the bottom slope and grain size of sediments). The cover of N. obtusa could be suppressed by shading and/or space limitation by dense stands of Potamogeton perfoliatus. Relatively low explained variation in the occurrence and coverage (respectively, 28 and 19%) of charophytes by environmental factors highlights the need for analysis of other important factors (e. g. chemical content of sediments, dynamics of propagule bank and herbivory), and better resolution in linking macrophyte community composition with environmental data.

Introduction

Coastal lagoons support diverse and productive communities, but have suffered impacts from anthropogenic and natural stressors (Kennish and Paerl, 2010). Coastal lagoons are listed as priority habitats (Code 1150, Habitats Directive (92/43/EEC)) and improvement of their ecological status is a focus of European Water Framework Directive (WFD, 2000/60/EC). The Directive promotes the assessment and monitoring of waters by the use of various biological quality elements, instead of relying primarily on chemical measurements. The quality elements outlined in the WFD for macrophytes are taxonomic composition and abundance. In the last decade, a large number of studies have aimed at developing indicators based on macrophytes for assessing quality and integrity of coastal lagoons (Christia et al., 2014 and reference therein).

Charophytes (Characeae) are among the species listed in the EU Habitat Directive as characteristic species of the coastal lagoons. Their dense beds provide shelter, food and substrate for benthic invertebrates, fishes and birds (Dugdale et al., 2006, Schmieder et al., 2006). Charophytes are generally recognized as indicators of good water quality (Pitkänen et al., 2013 and reference therein). However, according to the overview of WFD indicators (Birk et al., 2012), the relationship between indicators and pressure-impact was not tested empirically for about one-third of benthic flora indicators. The indicators for transitional waters have been the least validated. Only one indicator that includes metrics of charophytes was successfully tested for estuaries including tidal influenced freshwater sections (Steinhardt et al., 2009). Such a small number of validated indicators could be explained by the estuarine quality paradox, similarity between the features of organisms and assemblages in both estuaries and anthropogenically-stressed areas (Elliott and Quintino, 2007). Upper estuarine regions are a particular challenge as these exhibit large variability in salinity as well as nutrient and sediment loads.

To better understand the impact of anthropogenic stressors on charophytes in coastal lagoons, it is crucial to understand the response of charophytes along multiple environmental gradients. A number of recent studies have examined the influence of environmental factors on spatial and temporal patterns of charophytes in the Baltic Sea (e. g. Appelgren and Mattila, 2005, Steinhardt and Selig, 2007, Steinhardt and Selig, 2009, Rosqvist et al., 2010, Kovtun et al., 2011, Pitkänen et al., 2013, Torn et al., 2015, Blindow et al., 2016). Their results show that, salinity, eutrophication (light and nutrient gradients) and wave exposure (including bottom slope and substrate) were the key factors controlling the distribution of charophytes. These studies were mainly conducted in relatively sheltered coastal waters (boddens and lagoons), where there is typically low influence of freshwater. Studies examining both abiotic and biotic environmental factors are comparatively rare, particularly among the largest estuarine lagoons (e. g. Oder Lagoon, Vistula Lagoon, Curonian Lagoon).

The aim of the present study was to assess the importance of abiotic and biotic environmental factors controlling spatial patterns of charophyte occurrence and coverage in the temperate estuarine lagoon (Curonian Lagoon). Prior work in this system has shown that variability in riverine inputs and occasional wind-driven tidal forcing strongly affects salinity, water clarity and concentrations of nutrients in the Curonian Lagoon (Ferrarin et al., 2008). Thus, we hypothesized that these abiotic factors (especially near bottom salinity) should influence the occurrence and coverage of charophytes more than biotic interactions (i. e. coverage of aquatic angiosperms, width of reed belt). We also expected a relatively strong effect of water transparency and wave exposure.