Mart Simm

Ecological consequences of biological invasions: three invertebrate case studies in the north-eastern Baltic Sea

Population dynamics and ecological impacts of the cirriped Balanus improvisus, the polychaete Marenzelleria neglecta and the cladoceran Cercopagis pengoi were investigated in the north-eastern Baltic Sea. After an increase during the first decade of invasion, the density of M. neglecta and C. pengoi declined afterwards. The studied abiotic environmental variables did not explain the interannual variability in the seasonal cycles of M. neglecta and C. pengoi indicating that the species are at their initial phase of invasion. The population dynamics of B. improvisus was best described by water temperature. B. improvisus promoted the growth of the green alga Enteromorpha intestinalis. M. neglecta enhanced the content of sediment chlorophyll a and reduced growth and survival of the polychaete Hediste diversicolor and growth of the amphipod Monoporeia affinis. Concurrent with the invasion of C. pengoi the abundance of small-sized cladocerans declined, especially above the thermocline. C. pengoi had become an important food for nine-spined stickleback, bleak, herring and smelt.

Factors controlling long-term changes of the eutrophicated ecosystem of Pärnu Bay, Gulf of Riga

Phytoplankton, mesozooplankton, mysids and fish larvae were studied during 15–29 annual cycles measured weekly to monthly in Parnu Bay, the Gulf of Riga. The monthly variability of the biological data was related to temperature, ice conditions, salinity, influx of nutrients, the North Atlantic Oscillation (NAO) index, cloudiness and solar activity. Phytoplankton development was mainly a function of the NAO index. For the whole study period the abundance of zooplankton increased with increasing water temperature and solar activity. Significant correlations between phytoplankton and zooplankton densities were found until 1990. After the invasion of the predatory cladoceran Cercopagis pengoi in 1991, the zooplankton community was likely to be regulated by the introduced species rather than phytoplankton dynamics. The increased abundances of rotifers and copepods triggered the increase in mysid densities. The development of herring larvae was positively affected by the high density of copepods and rotifers but also by increased eutrophication. Until 1990 there was no significant relationship between the density of zooplankton and herring larvae. A negative relationship between the density of zooplankton and herring larvae in the 1990s suggests that the major shift in zooplankton community resulted in food limitation for herring larvae. The results indicated that (1) atmospheric processes in the northern Atlantic explain a large part of the interannual variation of the local phytoplankton stock, (2) trophic interactions control the development of pelagic communities at higher trophic levels, and (3) the introduction of an effective intermediate predator has repercussions for the whole pelagic food web in Parnu Bay.

Feeding ecology of pelagic fish species in the Gulf of Riga (Baltic Sea): the importance of changes in the zooplankton community

The feeding ecology of four pelagic fish species was studied in relation to their prey availability in the Gulf of Riga (Baltic Sea) during the summer 1999-2006. The zooplankton community was dominated by the cladoceran Bosmina longispina, rotifers Keratella cochlearis and K. quadrata and the copepod Eurytemora affinis, with the highest interannual variability in abundance recorded for B. longispina. The last influenced the diet of adult sprat Sprattus sprattus, juvenile smelt Osmerus eperlanus and three-spined stickleback Gasterosteus aculeatus as these were strongly selecting for B. longispina. The fish feeding activity did not match the abundance dynamics of their preferred prey, suggesting that fishes may switch to consume other prey in case the preferred diet was limited. A considerable dietary overlap indicated high potential competition between pelagic fish species. While herring Clupea harengus membras and G. aculeatus were relying on very different food, the diets of young O. eperlanus and G. aculeatus were very similar. Interannual variability in zooplankton composition and abundance significantly affected the diet composition of fishes, but those changes were insufficient to exert a consistent influence upon fish feeding activity and total amounts of zooplankton consumed.

Shifts in the Spring Herring (Clupea harengus membras) Larvae and Related Environment in the Eastern Baltic Sea over the Past 50 Years

Because of the high management relevance, commercial fish related aspects have often been central in marine ecosystem investigations. The iterative shiftogram method was applied to detect occurrence, type and timing of shifts in the single and multivariate time series linked to the spring spawning herring larvae in the Gulf of Riga (Baltic Sea). Altogether nineteen larval herring and related environmental variables were utilized during the period of 1957–2010. All the time series investigated, either single or multivariate, exhibited one or more shifts with variable type and timing. Multivariate shiftogram based on all time series identified two distinct states (1957–1983 and 1992–2010) in studied variables, separated by a smooth transition period lasting almost ten years. The observed shift was mainly related to hydroclimate and not to phenology or biota. Significantly increased variability was found in larval herring and recruitment abundances after the shift. While the shift in hydroclimate (1985–1991) was followed by the shift in phenology (1991–1997), the shift in biota occurred remarkably later (2003). It is likely that the dynamics in biota were affected by other drivers than those investigated in the current paper.

Temperature-driven changes in early life-history stages influence the Gulf of Riga spring spawning herring (Clupea harengus m.) recruitment abundance

Processes occurring during early life-history stages influence the year-class abundance of marine fish. We found that the abundance of 1-year-old spring spawning herring is statistically significantly determined by the number of post-flexion herring larvae in the Gulf of Riga (Baltic Sea). The abundance of consecutive developmental stages of larvae: yolk-sac, pre-flexion, flexion and post-flexion strongly correlated with each other, indicating that factors which already influence the yolk-sac stage are important in determining the abundance of post-flexion herring larvae. Winter air temperature before spawning determined the timing of maximum abundance of pre-flexion herring larvae, but not their main prey: copepod nauplii, implying that different mechanisms governing major preconditions for the formation of year-class strength. The abundance of post-flexion larvae displayed a potential dome-shaped relationship with sea surface temperature experienced after hatching. We suggest that increased summer temperatures, which exceed the physiological optimum negatively, affect the survival of post-flexion herring larvae. Overall, future climate warming poses an additional risk to larval herring survival and this may lead to a reduction in those herring stock which rely on recruitment from shallow coastal areas.

Winter–spring climate effects on small-sized copepods in the coastal Baltic Sea

The general positive effect of warmer winters on the abundance of small-sized zooplankton in the following spring and early summer has been reported from different parts of the Baltic Sea, but the mechanism of this link is not clear. Although causal links cannot be deduced with confidence from observational data, sufficiently detailed analyses can nevertheless provide insights to the potential mechanisms. We present an example of such an analysis, scrutinizing the effects of winter and spring hydroclimate on the abundance of small-sized dominant calanoid copepods (Eurytemora affinis and Acartia spp.), using data from 2080 zooplankton samples collected over 55 years (1957–2012) from a shallow coastal habitat (Parnu Bay, Gulf of Riga) in the Baltic Sea. Our results indicated that the milder winters brought about higher abundances, and reduced seasonality of small-sized copepods, whereas ambient sea surface temperature (SST) mostly affected the relative abundance of adult stages. The sliding window correlation tests revealed temporal shifts in the effects of controlling variables: with the continuous increase in SST, the effect of winter temperature on the abundance of Acartia spp. weakened. In contrast, E. affinis was consistently affected by SST, but the effect of winter temperature was more pronounced during the period of on average colder winters.