Temporal development of benthic invertebrate communities in the Baltic Sea from the 1960s to 2007

 
Figure: Examples of long-term changes in macrobenthic communities in the open-sea areas of the Baltic Sea, with inclusion of data from the Kattegat. Note differences in x-axes and groupings of taxa between the Kattegat and the other sea areas.

Assessment

When examining long-term trends in data collected between 1965 and 2007, it becomes immediately obvious that conditions were already disturbed in the mid-1960s. Benthic invertebrate status in the central parts of the Baltic Sea, in particular, is more or less entirely controlled by the presence or absence of hypoxia/anoxia. Current evidence suggests that the spatial and temporal extent of oxygen deficiency has increased over the past decades. In the light of historical work, it is also likely that reference conditions defined for open sea areas in this assessment are underestimates.

Generally, Baltic benthic macrofauna are characterized by small shallow-dwelling species owing to low salinity and transient hypoxia; historically it was only in the southern Baltic where more mature communities composed of deeper-dwelling, larger species, e.g. some long-lived bivalves and large polychaetes, could have developed. However, currently macrobenthic communities are severely degraded and below a 40-year average in the entire Baltic Sea.

Seasonal hypoxia, owing to increased nutrient inputs, has caused mortalities in the benthic communities in the Kattegat since the 1980s. The effects of hypoxia have been very patchy in both space and time, however, and cannot fully explain the general abundance pattern with high densities in the mid-1990s and relatively low values in the assessment period. Nor can hypoxia fully explain the long-term decrease in alpha species richness which occurred from the mid-1990s until 2006. The more wide-ranging implications of reduced benthic communities, hypoxia-induced or not, has been observed for demersal fisheries in this sea area.

In comparison with the Kattegat, benthic diversity is much reduced in the deep waters of the Arkona Basin, the Bornholm Basin and the southern Gotland Basin, owing to the lower salinity regime. Benthic community composition in this area co-varies strongly with oxygen-rich saltwater inflows from the Danish Straits. A shift towards more polychaete-dominated communities, which are more tolerant to eutrophication, has been observed in the Bornholm and Arkona Basins. It is often the polychaete Bylgides (Harmothoe) sarsi together with Scoloplos armiger that dominate the community at deep-water stations, while the bivalve Macoma balthica and the amphipod Pontoporeia femorata occur only when oxygen conditions improve. Anoxic conditions occur repeatedly below the halocline. The southern Baltic has experienced an overall reduction in salinity during the 20th century, which has resulted in a replacement of marine species with brackish-water species (BCSIII-10 in the Figure above). This also highlights the problem of setting reference conditions as baselines shift.

The benthic communities in the northern Baltic Proper and the northern and central parts of the Eastern Gotland Basin are seriously reduced. Owing to a permanent halocline and reduced oxygen conditions, this area had impoverished macrozoobenthic communities or anoxic sediment conditions during the 1970s and 1980s. Intermittently recovering communities were recorded in the 1990s. The saltwater inflow in 1993 strengthened the halocline, resulting in a lack of zoobenthic communities on approximately one third of this sea area. Bylgides sarsi is the most frequently occurring species at these deep stations (80–170 m), occasionally together with the bivalve Macoma balthica, the amphipods Pontoporeia femorata and the priapulid Halicryptus spinulosus. The response of opportunistic benthic species to improved oxygen conditions can be rapid, but with a delay in the recovery of total community abundance and biomass.

In the Gulf of Finland, generally low benthic community abundance, biomass and diversity were recorded below the halocline during the 1960s and 1970s. When the halocline weakened and disappeared because of the prolonged stagnation period from 1977–1993, this resulted in an increased oxygen content of the bottom waters and recovery of the macrozoobenthic communities. The halocline was re-established in 1993–1994 and the abundant macrobenthic communities recorded in the early 1990s in the deep central parts of the Gulf crashed almost completely in 1996–1997, and have not recovered to any larger extent owing to continued poor oxygen conditions. As the oxygen content of bottom waters is reduced, key species in the Gulf of Finland such as Monoporeia affinis and Pontoporeia femorata disappear, along with more resistant species such as Macoma balthica and Saduria entomon. The polychaete Bylgides sarsi is a fast colonizer in intermittently recovering areas.

In the Gulf of Bothnia, low salinity strongly reduces faunal diversity but also prevents the formation of water column stratification and hence makes conditions less susceptible to oxygen deficiency. However, in recent years some low oxygen levels (<40%) have been recorded, possibly due to early-stage eutrophication. Historically, macrobenthic communities have been entirely dominated by the amphipod Monoporeia affinis, which exhibits strong natural fluctuations in population abundance and usually comprises 70–100% of total community abundance. Abundances have been severely reduced since the peaks in abundance and biomass in the early to mid-1990s and are generally below the longterm average. The reasons for this decline are unknown. However, some recovery has been observed in certain areas of the Bothnian Sea during the past years. The invasive polychaete Marenzelleria spp. has spread rapidly throughout most of the Gulf of Bothnia. In the southern Bothnian Sea (station SR5), its abundances increased noticeably between 2004 and 2006 (when it comprised about 80% of total community abundance), but now polychaete numbers appear to be declining and the amphipods Monoporeia affinis and Pontoporeia femorata are recovering, at least in some areas of the Gulf.

References

Andersin, A.-B., J. Lassig, L. Parkkonen & H. Sandler (1978): The decline of macrofauna in the deeper parts of Baltic Proper and Gulf of Finland. Kieler Meeresforschungen 4:23-52. 

Hessle, C. (1924): Bottenboniteringar i inre Östersjön. Meddelanden från Kungliga Lantbruksstyrelsen, No. 250:12–29.

Karlson, K., R. Rosenberg & E. Bonsdorff (2002). Temporal and spatial large-scale effects of eutrophication and oxygen deficiency on benthic fauna in Scandinavian and Baltic waters - a review. Oceanography and Marine Biology: an Annual Review 40:427-289.

Laine, A.O., A.-B. Andersin, S. Leiniö & A.F. Zuur (2007): Stratifi cation-induced hypoxia as a structuring factor of macro zoobenthos in the open Gulf of Finland (Baltic Sea). Journal of Sea Research 57:65–77.

Laine, A.O., H. Sandler & J. Stigzelius (1997): Longterm changes of macrozoobenthos in the Eastern Gotland Basin and the Gulf of Finland (Baltic Sea) in relation to the hydrographical regime. Journal of Sea Research 38:135–159.

Norkko, A. & M. Jaale (2008): Trends in soft sediment macrozoobenthic communities in the open sea areas of the Baltic Sea. In: Olsonen, R. (ed.). MERI 62:73–80.

Norkko, A., T. Laakkonen & A. Laine (2007): Trends in soft-sediment macrozoobenthic communities in the open sea areas of the Baltic Sea. In: Olsonen, R. (ed.). MERI 59: 59–65.

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