Assessment of biodiversity and environmental quality using macrozoobenthos communities in the seagrass meadow (Gulf of Gdańsk, southern Baltic)

Seagrass meadows are among the most valuable and multifunctional habitats in the coastal zone of the Baltic Sea because of their engineering functions (Bonsdorff 1997; Boström et al. 2002; van der Heide et al. 2007). The macrozoobenthic biodiversity of such meadows is much higher than on the bare sand. Furthermore, it has been demonstrated that such meadows improve the environmental conditions (Bonsdorff 1997), even though very few small-scale studies assessing the environmental quality of one specific meadow have been carried out so far. Such studies would be very helpful in the context of the conservation and management of marine resources. The need to develop such strategies has been highlighted in recent decade, mainly because of the increased human impact (Borja et al. 2008). Indices based on macrozoobenthic communities, like the Benthic Quality Index (BQI) (Water Framework Directive 2000; Rosenberg et al. 2004), are among the tools used to monitor the environmental quality status. This makes meadows the most suitable areas for assessing and monitoring the environmental quality (Krause-Jensen et al. 2005), as any changes are immediately detectable. This applies not only to the meadows: they can also be used as reference plots for larger areas, even the whole water bodies.

The distribution range and taxonomic composition of the seagrass meadows in the southern Baltic have changed in recent years. Until the 1980s, their range gradually decreased (Węsławski et al. 2013), but since then a slow regeneration of vascular plants in the Gulf of Gdańsk has been observed.

The latest research shows that the seagrass meadows in the Gulf of Gdańsk are formed mainly by Zostera marina L., a protected species, but Zanichellia palustris L. and Potamogeton pectinatus L. are also abundant (Urbański et al. 2009; Boström et al. 2014). Apart from their biological value, for example, as a shelter for animals and a complex habitat for many species, such meadows are also of economic importance: they are on every diver’s list of water areas to be visited, and as an educational example they are second to none. One of these meadows on Dluga Mielizna is additionally protected as part of the Natura 2000 Special Area of Conservation (SAC) PLH220032 (Puck Bay and the Hel Peninsula) and Special Protection Area (SPA) PLB220005 (Puck Bay). As a coastal habitat, it is also situated within the Coastal Landscape Park.

In environmental quality assessments, it is very important to obtain unequivocal results regarding a region. Depending on the features tested and the indices used in the Gulf of Gdansk, its environmental status varies from good to bad (e.g. HELCOM 2009; CIEP 2010; RIEP 2013, Saniewski 2013; HELCOM 2014). Only large monitoring research projects or studies strictly related to sandy bottom indices were carried out (e.g. Osowiecki et al. 2008; 2012; Šiaulys et al. 2011). None of the subsequently published papers describe the benthic fauna inhabiting the seabed covered with macrophytes. Such research usually focuses on soft sea beds which, even though they are the most common type, are not as multifunctional as seagrasses. No studies to date have linked the environmental status with the presence of seagrass. This is only mentioned in papers when environmental quality is being assessed on the basis of the state of macrophytes or when the depth limits for plants are being investigated (Krause-Jensen et al. 2005; Saniewski 2013).

The objectives of this study were to describe the taxonomic diversity of macrofauna in the seagrass meadows on Dluga Mielizna, to show the influence of vascular plants on the diversity, and to compare the macrozoobenthic diversity on two types of seabed: sand covered with vascular plants and bare sand. An additional objective was to define the environmental status of this meadow and to show how the presence of vascular plants make it superior to a bare sandy bottom.

Z. marina is very sensitive to eutrophication and environmental changes (Boström et al. 2014), so its presence on Dluga Mielizna can be regarded as an indicator of good quality. In some samples, leaves with seeds were found, so the seagrass meadow on Dluga Mielizna can be regarded as well-formed and valuable. The low abundance of fi lamentous algae on Dluga Mielizna, probably due to animal grazing, is also helpful in developing the good quality of the meadow.

With regard to the taxonomic diversity of macrozoobenthos, the number of taxa recorded in this study (33) was very high for such a limited area. This is the same number of taxa as that found in the Puck Lagoon (Legezynska & Wiktor 1981; Janas & Kendzierska 2014). In their study of seagrass meadows, Janas & Kendzierska (2014) found only 20 taxa, while Kotwicki (1997) reported only 13 taxa in the coastal waters off Gdynia where seagrass meadows may occur. On the other hand, the few studies conducted only on the sandy bottom in the coastal waters of Puck Bay all report a lower number of taxa, ranging from 15 to 25 (Herra & Wiktor 1985; Kotwicki 1997; Kotwicki et al. 1999). The dominance of Hydrobiidae observed in these studies is typical of sandy bottoms at this depth (Bick & Zettler 1994; Boström & Bonsdorff 1997; Osowiecki 1998; Kotwicki et al. 1999; Włodarska-Kowalczuk et al. 2010). This shows that the seagrass meadow is a complex habitat, suitable for various kinds of animals, not only those associated with plants. Lavesque et al. (2009) demonstrated a high level of diversity in the seagrass meadow in Arcachon Bay – 30 taxa – which is very similar to that in Puck Bay.

Studies of the sandy bottom show that the waters off Kuznica (close to our study area) are among the species-richest areas of the six areas investigated by Osowiecki (1998) in the Gulf of Gdansk. The abundance reported in the study by Osowiecki ranged from 12000 to 13500 ind. m^-2. These figures correspond with those at many stations on Dluga Mielizna in 2008. One of the reasons for this situation is the stabilization of environmental conditions due to the presence of seagrass and the protection of animals provided by leaves, e.g. against strong water currents (Boström & Bonsdorff 2000). There were 13 taxa living exclusively among seagrasses. Comparing these results with the study conducted by Smola (2012) in a seagrass meadow near Gdynia, four of these 13 taxa were present only on the seagrass on Dluga Mielizna; the four other taxa were also recorded by Smola. We can conclude that this meadow is also more valuable than other meadows in Puck Bay, due to the occurrence of very rare species, such as I. balthica and G. inaequicauda.

The uniqueness of these 13 taxa on the vegetation-covered bottom is corroborated by the research of Boström & Bonsdorff (1997), where taxa such as T. fluviatilis, E. crustulenta or Idotea spp. were found only in the meadows, and not on the bare sand. Among the plants, Kendzierska et al. (2014) found a species of leech (P. pojmanskae), described in the Gulf of Gdansk for the first time and not observed in other areas. However, there was a small number of non-indigenous taxa in our studies. The fact that there were only four of them (Marenzelleria spp., A. improvisus, G. tigrinus and M. arenaria), compared to eight taxa reported in the study by Janas & Kendzierska (2014), demonstrates the natural character and good stability of this habitat.

Similar differences between the underwater meadow and the bare sandy bottom, demonstrated in this work, have been also found in other parts of Europe, for example in Norwegian waters (Frederiksen et al. 2010). Those authors found twice as many organisms on the vegetation-covered seabed compared to the sandy bottom. The number of species off the coasts of Norway (113) is much higher than in Puck Bay, but at least 35 of them were represented by a single specimen.

It can be concluded that the environmental status of Dluga Mielizna is good. This result is rather better than that obtained in other researches, e.g. Korpinen et al. (2013), which shows that the whole Gulf of Gdansk is an area subject to moderate or strong human impact, especially the bottom habitats. In this case, the importance of seagrass as an indicator is even greater, because it can be regarded as an indication of good habitat quality. It may be objected that seagrasses and animals living among them are not good indicators for this area, because the bay freezes over every winter. However, Jankowska et al. (2014) showed that the meadows in the Gulf of Gdansk are among those that do not decay during winter, so they are present throughout the year.

All the above arguments show that the presence of vascular plants is essential for good environmental quality. They not only enhance the complexity and beauty of the seascape, but also the biodiversity of species living among the leaves and in the sediment. Bare sand as a habitat is more likely to be destroyed and fragmented, for example, by an unexpectedly powerful wave, which renders such an unstable habitat vulnerable to sudden changes. Many studies have shown that habitats with vascular plants are much more valuable: they have a higher degree of biodiversity with greater abundance and environmental quality (Boström & Bonsdorff 1997; 2000). A more stable and diverse habitat with more species makes seagrass meadows more useful for environmental quality research than bare sand. This study has demonstrated that seagrass meadows are inhabited by diverse benthic communities with many crustaceans that are sensitive to disturbances. Changes in their environment can thus be more easily monitored than in the benthic communities on the bare sand. These results can be very useful for further studies, either for comparison with other regions of the Baltic, or as historical, reference data for future work in the same region.

The taxonomic diversity of macrozoobenthos in the seagrass meadow on Dluga Mielizna is very high (33 taxa). There are also statistically significant differences in the number of taxa, abundance and biomass as well as the Shannon and Benthic Quality Indices between bare sand and bottom covered with vascular plants in favor of seagrasses.

The environmental quality of Dluga Mielizna, assessed using the benthic fauna, is good.