Trends in soft sediment macrozoobenthic communities in the open sea areas of the Baltic Sea (1965 to 2005)
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Key messages
Macrobenthic communities are severely degraded throughout the Baltic Proper and the Gulf of Finland and are below the longterm average. Populations of the amphipod Monoporeia affinis have crashed in Gulf of Bothnia and the invasive polychaete Marenzelleria viridis has spread.
Results and assessment
Relevance of the indicator for describing developments in the environment
Soft-sediment macrobenthic communities are central elements of Baltic Sea ecosystems and provide excellent indicators of environmental health. Most macrobenthic animals are relatively long lived (several years) and thus integrate changes and fluctuations in the environment over a longer period of time. Variations in species composition, abundance and biomass can be used to assess environmental disturbance. However, while comparatively rich and diverse shallow-water benthic communities are amenable for more sensitive analyses of eutrophication effects, the deeper water benthic communities, below the halocline, are comparatively less diverse and naturally exposed to hypoxic disturbance events due to the characteristics of the Baltic Sea and the influence of episodic saltwater intrusions. This feature highlights the role of the frequency and intensity of inflow events in shaping deeper water zoobenthic communities. Without a question, eutrophication has exacerbated the intensity of naturally occurring hypoxic events. Long-term monitoring and knowledge of the natural fluctuations in the environment are critical for understanding the dynamics of the systems and interpreting possible anthropogenic changes. In this regard, monitoring of zoobenthos provide some of the best available biological long-term data available in the Baltic Sea.
In the Baltic Proper and the Gulf of Finland the macrobenthic communities in areas below the permanent halocline (60-80 m) are strongly dependent on the physical conditions in the area (e.g. saline and fresh water inflows, length of stagnation periods). In these areas the benthic communities react most strongly to changes in water oxygen content. In areas above the permanent halocline, changes in community structure as well as increasing trends in macrobenthic production indicate eutrophication.
In the Gulf of Bothnia low salinity prevents the formation of water column stratification and oxygen deficiency, and also strongly reduces faunal diversity. In deeper soft bottoms the crustacean, Monoporeia affinis, is the numerical dominant comprising 70-100% of total community abundance. The populations show more or less cyclic fluctuations in all parts of the area, due to natural variations in the population structure. As detritus feeders these populations are dependent on the pelagic production and its sedimentation. Thus changes in the input of organic material are mirrored in the populations of this species.
Due to difficulty in defining historic reference conditions we here emphasise the importance of considering the state and current trends in macrobenthic community development in relation to a long-term average. In combination with the rapid assessment of (Fig. 1).
Figure 1. State and trends of macrozoobenthic communities in open sea areas of the Baltic Sea. The map illustrates sites sampled during the annual COMBINE 2 cruise by R/V Aranda and the on-board rapid assessment of the state of macrobenthic communities in 2005 (as indicated by community abundance). Trends in community abundance over the past 40 years are illustrated at selected sites in different sub-regions of the Baltic Sea; curves represent 5-year moving averages and the dotted lines indicate long-term averages (based on entire study period). The x-axes depict years and the y-axes number of individuals per m2. Note differences in scale on y-axes. Click image to enlarge!
To see the charts larger, click links below:
BO3, CVI, US6b, LL4a, LL6a, LL11, BCSIVB6, BCSIII-10, LL12, SR5
Policy relevance and policy references
Initiatives to reduce nutrient loads are central to reducing biological oxygen demand that exacerbates the formation of hypoxic events. Defining reference conditions is difficult due to the strong dependence of macrofauna on oxygen concentrations and naturally occurring stratification and hypoxia, highlighting the need to gauge current trends in the state of macrobenthic communities against long-term time series.
Assessment
In the Baltic proper the macrobenthic communities below the permanent halocline have totally disappeared or become utterly impoverished because of oxygen deficiency caused by the saltwater inflow and subsequent strengthening of the halocline in 1993-1994. At present the area devoid of macrofauna is of the same size as during the middle of the last stagnation period in the 1970s and 1980s, i.e. about one third of the total sea area. In the Gulf of Finland the salinity stratification disappeared during the prolonged stagnation and lack of saltwater inflows during 1977-1993, but was re-established in 1993-94. The abundant macrobenthic communities recorded in the early 1990s in the deep central parts of the Gulf crashed almost completely in 1996-97, and have not recovered to any larger extent due to continued poor oxygen conditions below the permanent halocline. The latest larger saltwater inflow in 2003 resulted in some colonization by opportunistic species in the Gotland Basin area, but oxygen conditions have since deteriorated and in 2005 conditions were yet again anoxic and macrofauna more or less eliminated (Figs 1 & 2, Table 1). Conditions are below the all time average and in a very poor state (Fig. 1).
Figure 2. Long-term changes in species composition at selected sites in different sub-regions of the Baltic Sea (see Fig. 1 for position of sites). The x-axes depict years and the y-axes number of individuals per m2. Note differences in scale on y-axes. Arrows indicate years when no samples were obtained. Click image to enlarge!
In the Gulf of Bothnia macrobenthic communities are entirely dominated by the amphipod Monoporeia affinis, which show strong natural fluctuations in population abundance (Fig. 2). Abundances have been dramatically reduced since the peaks in abundance in the early to mid 1990’s (Fig. 1) and are now well below the long time average (Fig. 1). The reasons for this decline are unknown. In 2005 the invasive polychaete Marenzelleria viridis was found at 6 of 10 sampled stations and in the southern Bothnian Sea (station SR5), abundances had increased noticeably since 2004 (> 18% of total community abundance in 2004), illustrating the polychaetes strong establishment in the area.
Table 1. The number of sites sampled in respective sub-regions of the Baltic Sea in 2005, their mean depth and the average state of benthic communities at the sampled stations (derived from the yearly on-board rapid assessment) is given and the net change from 2004 is also indicated. Rapid assessment scale: 0 = no fauna, 1 = severely reduced, 2 = moderate fauna, 3 = abundant (see also Fig. 1).
References
Andersin A-B, Lassig J, Parkkonen L, Sandler H (1978a) Long-term fluctuations of the soft bottom macrofauna in the deep areas of the Gulf of Bothnia 1954 – 1974, with special reference to Pontoporeia affinis Lindström (Amphipoda). Finn Mar Res 244: 137-144.
Andersin A-B, Lassig J, Parkkonen L, Sandler H (1978b) The decline of macrofauna in the deeper parts of Baltic proper and Gulf of Finland. Kieler Meeresforsch 4: 23-52.
Karlson K, Rosenberg R, Bonsdorff E (2002) Temporal trends and spatial large-scale effects of eutrophication and oxygen deficiency on benthic fauna in Scandinavian and Baltic Waters. Ocenaogr Mar Biol Annu Rev 40:427-489.
Laine AO, Sandler H, Andersin A-B, Stigzelius J (1997) Long-term changes of macrozoobenthos in the eastern Gotland Basin and the Gulf of Finland (Baltic Sea) in relation to the hydrographical regime. J Sea Res 38: 135-159.
Rumohr H, Bonsdorff E, Pearson TH (1996) Zoobenthic succession in Baltic sedimentary habitats. Arch Fish Mar Res 44: 179-214.
Metadata
Technical information
Data source: Data on macrobenthic community composition was obtained during monitoring cruises of the FIMR since 1964-65. The data produced by FIMR is kept at the database of the FIMR. At FIMR contact persons are Jan-Erik Bruun and Alf Norkko.
Description of data: The quantitative data are mean values of 3-5 parallell samples (sampler area 0.1 m2), multiplied to values per square meter. The data are collected within the framework of the HELCOM COMBINE programme. The rapid assessment data is determined onboard and facilitates the detection of broadscale change.
Geographical coverage: All regions of the Baltic Sea except for the Gulf of Riga and the western Gotland Basin.
Temporal coverage: From 1965 to 2005.
Methodology and frequency of data collection: Sampling and analyses are made according to the guidelines for the HELCOM COMBINE programme. Sampling is performed once a year in May-June.
Quality information
Strength and weakness: Due to the longevity of macrobenthic animals they provide an excellent indicator of environmental health.
Reliability, accuracy, robustness, uncertainty (at data level): Interpretation of the data needs to be done based on long-term data.
Further work required (for data level and indicator level): The indicator will be updated annually, both in terms of the rapid assessment and the quantitative data. Due to the labour required for sorting macrofaunal samples, the quantitative data lags behind with one year.
For reference purposes, please cite this indicator fact sheet as follows:
[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2005. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
Last updated 25 Nov 2005.