WaterSubject

The European water framework directive (WFD) aims at achieving good ecological status of water bodies, which implies assessment of their current ecological quality status in respect to defined reference (pristine) conditions. In this paper, two historical biodiversity datasets (from 1920s and 1950s) and results from the recent inventory are used to trace the long-term changes of the macrozoobenthos in the eutrophic boreal lagoon of the Baltic Sea.

In comparison of datasets the highest congruence was obtained for molluscs and malacostracan crustaceans, which also had a similar level of taxonomic emphasis between studies. Considering inconsistencies in methodology and taxonomic determination, only few species extinctions in these groups did likely occur during the last 100 years. Two amphipod species (Gammarus pulex and Gammarus lacustris) were not found during the recent survey, whereas five new species of this taxonomic group occurred in the lagoon since 1950s. The causes of these extinctions remain unclear; however displacement by established new amphipods cannot be excluded. Theodoxus fluviatilis was recently recorded in the very restricted area of the lagoon, while in earlier studies the species was mentioned as common and widely distributed in the water body. On the other hand, 10 gastropod species and 9 bivalves were reported for the first time in the lagoon and most likely have been overlooked in earlier surveys. Approximately 10% of the species have their origin outside the Baltic Sea basin and the number of invasions considerably exceeds the number of likely extinctions. Assessment scheme of such changes is unclear following WFD guidelines, therefore elaboration of a framework for evaluation of the alien species diversity in a context of local biodiversity should attain more effort when implementing the WFD.

Enclosed water bodies (e.g. bays, coastal lagoons, etc.) have undergone drastic changes during the last decades. Being directly impacted by processes in the drainage basin, many of these systems in the Baltic Sea (e.g. Stettin Lagoon, Puck Lagoon, Vistula Lagoon, Curonian Lagoon and Gulf of Riga) have been enriched with nutrients. Rate of species introductions was also increasing during the last century and resulted in structural and functional changes of local communities.

The European water framework directive (WFD) establishes a framework for the protection of all waters and aims at achieving good ecological status. One of its main tasks is the classification of water bodies into different types assessing their ecological quality status in respect to defined reference (pristine) conditions. Reference conditions for biological parameters could be defined using modelling approaches, historical datasets and existing information on not impacted sites or expert judgement. However, historical information containing biodiversity list is usually the only source of data on the status of water bodies for periods long time ago.

Here we test the use of historical biodiversity datasets in tracing the long-term changes of macrozoobenthos diversity in the eutrophic boreal lagoon. We present results of an extensive faunistic inventory of major macrozoobenthic groups of the Curonian Lagoon. Then, these results are compared with two historical datasets originating from the beginning and the middle of the 20th century.The present study was focused on the macrozoobenthos diversity in the largest lagoon of the Baltic Sea. Due to the high substrate variability and the high freshwater input from the Nemunas River, the Curonian Lagoon belongs to one of the most macrozoobenthos diverse estuaries of the Baltic Sea. Investigated littoral habitats showed considerably higher species diversity than soft bottoms in the open lagoon. Long-term monitoring and other surveys performed during the period from 1980 to 2000 reported 85 species altogether (oligochaetes, chironomids and insects excluded) in the open parts of central and northern lagoon. All organisms identified to the species level were also found in our survey and most of them have been determined as common in the lagoon. Similarly, in 5 littoral stations in the Strait area and adjacent waters we found 31 macrofauna species (excluding oligochaetes and chironomids), whereas 7 of those only have been mentioned after visiting 30 sites 3 times in the same year. On the other hand, during seasonal sampling, these authors found some accidental brackish water species (Mytilus edulis, Macoma balthica, Praunus inermis, Crangon crangon, Euridice pulchra, Idotea chelipes), which we did not observe. Generally, this indicates that our littoral survey covers well the species, which permanently inhabit the lagoon; however, it may overlook taxa (originally from the marine waters off the estuary) occurring in the lagoon sporadically.

Three potential sources of differences in the results from three time periods can be identified: (1) different sampling efforts and methods, (2) differences in level of emphasis on distinct zoological groups and (3) species extinctions and introductions.

Sampling efforts and methods certainly differed between our and historical studies. It is unclear, how samples have been collected and sorted in the beginning of the 20th century (only dredging and observations were noted) as well as number and locations of stations remain unknown. However, according to the description, sampling sites were restricted to the western coast of the lagoon and the number of littoral sites visited along the sea–river gradient was most likely lower than in both later studies. In contrast to Szidat’s data, results published by originated from the very extensive survey carried out in the entire lagoon using Ekman-Birge and Petersen grabs as well as benthic corer. Out of 213 stations approximately 60 were located in the lagoon’s area, which has been also covered by our stations. Although many species were described from a littoral part and the author most likely visited numerous places, number of these shallow stations remains unclear. It can be concluded that, according to the efforts spent in sampling, Szidat’s study can be considered as a very first compilation of sporadic results by the time. Sampling effort was considerably lower than in two later studies; therefore, most likely widespread and characteristic species (particularly for groups of molluscs and crustaceans) did occur in samples only. In contrast, comprehensive species list from 1950s exhaustively reflects lagoon’s biodiversity status 50 years ago. Many sampling sites were located in the open part of the lagoon, whereas grid of shallow sampling sites is comparable with that of our study. Since littoral stations are more efficient in covering biodiversity, we do no anticipate major differences between these two datasets due to sampling design.