In Situ Production of Charophyte Communities under Reduced Light Conditions in a Brackish-Water Ecosystem

Abstract

INTRODUCTION Charophyte communities are an important element in shallow enclosed fresh- and brackish-water ecosystems (Mathieson and Nienhuis, 1991; van den Berg et al., 1998; Pelechaty et al., 2006). They provide shelter and habitat for numerous species including epiphytic microalgae, filamentous macroalgae, as well as various crustacean and insect species (Linden et al., 2003; Schmieder et al., 2006; Torn et al., 2010). Besides, charophytes are an important component in the food web as part of the diet of benthic invertebrates (Kotta et al., 2004, 2013), waterfowl (Noordhuis et al., 2002; Schmieder et al., 2006), and fish and fish larvae (de Winton et al., 2002; Dugdale et al., 2006). Declining distribution and diversity of charophytes have been observed in many regions worldwide including the brackish Baltic Sea (Blindow, 2000, 2001; Schubert and Blindow 2003; Munsterhjelm, 2005). Eutrophication is assumed to be the most important threat to charophytes causing their decline (e.g. Blindow, 1992; Auderset Joye et al., 2002). The main effect associated with eutrophication is the bloom of ephemeral planktonic algae, which leads to increased sedimentation, water turbidity and, as a result, reduced light availability. The shortage of light may reduce the photosynthetic production and growth of charophytes down to the level where their sustainable development becomes impossible (Blindow et al., 2002; Johnsen and Sosik, 2004; Hautier et al., 2009; Dickey et al., 2011). On the other hand, charophytes often prefer soft bottom habitats where even moderate wind may cause sediment resuspension and sedimentation of particles on the plant surface. In such habitats underwater light climate is naturally very variable (Schneider et al., 2006 and references therein). Thus, charophytes are adapted to periodic stress of low light intensities. Nevertheless, the interactive effect of elevated eutrophication and weather variables may result in poorer light conditions than expected from their separate effects (Blindow et al., 2003; Kling et al., 2003). So far, the studies concerning photosynthesis of charophytes are mainly based on laboratory experiments with either detached pieces or single individuals (e.g. Blindow et al., 2003; Marquardt and Schubert, 2009). Very few have been carried out in the natural environments, especially in brackish bodies of water. As compared to their freshwater counterparts, charophytes are often naturally stressed at elevated salinity and therefore are expected to respond differently to changes in light conditions (e.g. Blindow et al., 2003). The existing data on in situ primary production of charophytes related to light limitation are scarce and hardly comparable because of difference in methodologies and the environmental conditions among habitats (Kufel and Kufel, 2002). Light is a key limiting factor for photosynthetic production in aquatic environments (Kurtz et al., 2003; Asaeda et al., 2004, Binzer et al., 2006; Zhang et al., 2010). Earlier experimental studies carried out at the community level have also shown that canopy density and canopy structure significantly affect the photosynthetic production of marine macroalgae (Middelboe et al., 2006). This suggests that macroalgal communities are largely light-limited and such light limitation increases with canopy height and/or community biomass (Parnoja et al., 2013). Altough the photosynthetic production of marine macroalgae at the community level has been increasingly studied (Middelboe and Binzer, 2004; Middelboe et al., 2006; Parnoja et al., 2013), to the best of our knowledge, there is only a single study on charophyte communities (Libbert and Walter, 1985). Based on the above, our goal was to determine the primary production of a charophyte community under manipulated in situ light conditions. We hypothesized that (a) the community would have higher responses under more severe light limitation and (b) the recovery of charophyte photosynthetic performance would be faster under less severe disturbances. …