Abiotic factors

The ecosystem changes in the Baltic Sea are synthesized by the ICES WGIAB (2008 and subse-quent reports) in Integrated Ecosystem Assessments (IEA) conducted for seven subregions of the Baltic Sea: i) the Sound (ÖS), ii) the Central Baltic Sea (CBS), encompassing the three deep basins, Bornholm Basin, Gdańsk Deep and Gotland Basin; iii) the Gulf of Riga (GoR), iv) the Gulf of Finland (GoF), v) the Bothnian Sea (BoS), vi) the Bothnian Bay (BOB) and a coastal site in the southwestern Baltic Sea (COAST). The updated IEA (ICES WGIAB, 2015) corroborated the cor-relation between temperature and salinity, and included 2014 values for the abiotic factors being tracked.

The main drivers of the observed ecosystem changes vary somewhat between subregions, but they all include the increasing temperature and decreasing salinity (Figure 1.2). These are influ-enced by large-scale atmospheric processes illustrated by the Baltic Sea Index (BSI), a regional calibration of the North Atlantic Oscillation index (NAO) (Lehmann et al., 2002). The change from a generally negative to a positive index for both BSI and NAO in the late eighties was associated with more frequent westerly winds, warmer winter and eventually a warmer climate over the

area (Figure 1.2). Further, the absence of major inflow events has been hypothesized to be related to the high NAO period (Hänninen et al., 2000). An indication of this is that only two major in-flows to the Baltic Sea have been recorded during the high BSI-period since the late 1980s. Con-trary to what occurred in surface waters, salinity in deeper waters has increased after the early 1990s to levels as high as in 1960s–1970s (Figure 1.2). However, Mohrholz (2018) concluded that

“until today climate change has no obvious impact on the MBI related oxygen supply to the central Baltic Sea. The increased eutrophication during the last century is most probably the main driver for temporal and spatial spreading of suboxic and anoxic conditions in the deep layer of the Baltic Sea.“

Figure 1.2. Time-series in summer surface temperature and surface salinity (top panels), BSI (Baltic Sea Index) and NAO (North Atlantic Oscillation index) and deep salinity (lower panel) in the Gotland Basin and Bornholm Basin.

In addition to temperature and salinity, fishing pressure was identified as an important driver for CBS and BoS. For the highly eutrophicated GoF, also nutrient loads were found to be an important driver. Trends in nutrient concentration and loading vary between the subregions; the concentrations of DIN and DIP decreases in ÖS and CBS, whereas in GoR and GoF DIP concen-tration is increasing because of internal loading. In contrast, in BoS and BoB DIN concenconcen-tration is increasing, and in BoB and COAST the total DIP loading from run-off is also increasing. Alt-hough the long-term decrease in salinity is apparent in all subregions, the recent trends in salinity differ. In GoR, as in the CBS, salinity in the surface layer has increased since 2003, whereas in COAST salinity is continuing to decrease due to the increased freshwater input from run-off.

The suggested driving forces of the observed regime shift in all subregions, decreasing salinity and increasing temperature, are both consequences of climate change. However, it must be un-derlined that the population changes observed in several trophic levels (fish and plankton) in many areas are also the result of top–down regulation and trophic cascades (Casini et al., 2008, 2009), emphasizing the role of fishing pressure on ecosystem changes.

Moreover, the reversal of abiotic factors back to the values as observed in the 1970s–1980s did not produce a parallel reversal of the biotic conditions, this likely confirming that currently the Baltic Sea is strongly controlled by other mechanisms, as for ex. trophic interactions (Casini et al., 2009, 2010; Möllmann et al., 2009).

Contaminant levels in general remain elevated, and the overall contamination status has been at the same level for the past two decades, but many potential contaminants are not monitored.

Some of the main contaminants have been reduced (e.g. DDT, dioxins, and PCBs).

A particular feature of the Baltic Sea since the mid-1990s has been a drastic increase in the extent of anoxic and hypoxic areas and potentially increased biological oxygen consumption on seafloor (Figure 1.3).

Figure 1.3. Time-series of anoxic and hypoxic seabed in the entire Baltic Proper. From the Swedish Meteorological and Hydrological Institute (SMHI) annual report.

The underlying processes leading to a certain stock status and furnishes an easy-to-understand way to communicate the results to the stakeholders and managers (Working Document 6 in the WGBFAS 2010 report). The approach has recently been further developed to provide a visually effective way to track changes in the performance of drivers of fish stock dynamics (Eero et al., 2012). In a changing environment, the status of individual fish populations and consequently the fishing possibilities can change rapidly, not always for reasons directly related to fisheries. In order to take the ecosystem context into account in the management process and achieve con-sensus concerning fishing possibilities among stakeholders, it is important that the status of var-ious drivers influencing fish stocks, and their relative impacts are broadly understood.

An overview of the dynamics of the eastern Baltic cod, sprat and central Baltic herring SSB and recruitment together with the dynamics of drivers influencing the dynamics of biomass and re-cruitment is presented in Figure 1.4.

Environmental conditions for Eastern Baltic cod recruitment of year classes 2010–2011 were as-sessed by the ICES/HELCOM Working Group on Integrated Assessments of the Baltic Sea (ICES WGIAB, 2013). This assessment was made based on an indicator of the limiting abiotic conditions for cod egg survival, the reproductive volume, found to be the most encompassing indicator of the significant indicators of environmental conditions of cod recruitment (as assessed by models on SSB-recruitment residuals; WGIAB, 2013). The reference value of reproductive volume dis-tinguishing positive from negative environmental influence on cod recruitment (Figure 1.5) was derived using the quantitative relationship between recruitment residuals and reproductive vol-ume (WGIAB, 2013).

Figure 1.4. Temporal changes in indicators influencing the SSB and recruitment of the eastern Baltic cod, sprat and central Baltic herring. The colours refer to quartiles of the values observed in the time-series, high values are marked with blue and low values with red colours, except for mortality where the colours are inversed. The lines show the trends in SSB and Recruitment of the stocks, the dost for recruitment in the final years show the values used in short-term forecast (R-recuitment; w-weight-at-age; land-landings, f-fishing mortality-at-age; M-natural mortality (average of ages 1–7);

S100_GB- salinity at 100 m depth in Gotland Basin; COD_RV- cod reproductive volume, Pseudo_Spr-abundance of Pseu-docalanus in spring; T-BB-60_spr- temperature at 60 m depth in spring in Bornholm Basin; SST_BB_Sum- Sea surface temperature in summer in Bornholm Basin).

Figure 1.5. Time-series of reproductive volume for Eastern Baltic cod (summed across the three deep basins in the Baltic Sea), assembled by WGIAB 2013. Relationships between each variable and residuals from cod recruitment (back shifted) vs. cod SSB were derived during WGIAB 2013, using linear models of first or second-order polynomials for year classes 1977–2009. Bars indicate the values relative to the reference value of each variable (derived from the fitted relationships on cod recruitment residuals, as the point where there is no environmental effect on recruitment); green bars indicate beneficial environmental conditions and red bars poor conditions for cod egg survival. This shows the poor conditions for cod recruitment for the year classes 2010–2011 (corresponding to recruitment of age 2 in 2012–2013).

SSB Recruitment