Changes in northern Baltic zooplankton and herring nutrition from 1980s to 1990s:top-down and bottom-up processes at work

Abstract

During the stagnation period of the Baltic Sea the mean weight-at-age of Baltic herring decreased by 50% (between 1977 and 1992). This has usually been attributed to a top-down process, i.e. to the simultaneous collapse of cod stocks and their predation. We present long-term data for 1980 to 1993 showing that bottom-up effects may also have played a role: along with the decline of salinity. the biomass proportion of zooplankton taxa preferred by herring (larger than 20 pg ind:' in wet welght) significantly declined. To support our hypothesis we present a study in which Baltic herring feeding and selective predation were investigated during 1985, a time of good growth and high weightat-age, and 1991, when herring growth and weight-at-age were poor. In this study, herring ston~achs and simultaneously taken plankton samples were analysed from trawl surveys conducted in the northern Baltic proper d'uring the peak of the herring feeding season in late summer. During both 1985 and 1991, herring selectively preyed on the larger zooplankton categones, especially neritic copepods. However, in 1991, a smaller proportion of the prey in herring stomachs consisted of neritic copepods, apparently because their share in plankton had decreased. Consequently, and despite an increase in total zooplankton biomass, the estimated carbon content of the food eaten by herring was lower, and the average stomach fullness index (on a scale of 0 to 5) decreased from 3.9 in 1985 to 1.9 in 1991. Also, the amount of mesenteric fat on herring stomachs declined from 4.2 to 3.2 (scale 0 to 5), indicating a longer-term failure in feeding success. We suggest that, in addition to possible top-down effects (a release of cod predation), bottom-up processes mediated via changes in mesozooplankton species composition have also influenced hernng growth and that both of these processes are affected by the same environmental factor-the Baltic salinity level.

Sedimentation of copepod fecal material in the coastal northern Baltic Sea: Where did all the pellets go?

“…This effect seemed to balance or override other potentially important factors, such as seasonal and spatial variations in microbial activity or food concentration (Chl a). Therefore, significant changes in sedimentation rates of copepod fecal material may occur along with long-term changes in zooplankton community composition, which commonly occur in the northern Baltic Sea (e.g., Viitasalo et al 1995b;Flinkman et al 1998;Vuorinen et al 1998). …”

Food consumption by clupeids in the Central Baltic: evidence for top-down control?

“…Investigations on mesozooplankton abundance and species composition in the Central Baltic Sea have revealed a shift from larger to smaller copepods and cladocerans in the Baltic Proper (HELCOM, 1996) and in the Gulf of Finland (Flinkman et al, 1998). These changes have so far not been related to top-down control effects, but to decreased salinity caused by lack of inflows of saline water from the North Sea (Matthäus and Franck, 1992) and exceptionally high rain fall (Bergströ m and Carlsson, 1994).…”

Adaptation strategies of copepods (superfamily Centropagoidea) in the White Sea (66°N)

“…In the northern Baltic, females are observed all the year round, grow up to 880 m PL and contain up to 1.056 g C (Tanskanen 1994). As the northern Baltic, like the White Sea, is characterized by lower salinity compared to oceanic values (Flinkman et al 1998), we consider that the White Sea population of Acartia biWlosa exhibits similarity to the northern Baltic population, which nicely coordinates with the Baltic origin of this species in the White Sea about 6,000 years ago (Pertsova and Pantyulin 2005). In Southampton Waters, A. biWlosa prefers a temperature span of +5.0 to +12.0°C and is observed from January till June, producing both diapausing and subitaneous eggs (Castro-Longoria and Williams 1999).…”