ALBERT

Description: Long-term dynamics (1959–1997) of the copepod species Pseudocalanus elongatus, Temora longicornis, Acartia spp. and Centropages hamatus, as well as the taxonomic group of cladocerans, are described for the open sea areas of the central Baltic Sea. Differences between areas, i.e. Bornholm Basin, Gdansk Deep and Gotland Basin, as well as between 5 year periods, were investigated by means of Analysis of Variance (ANOVA). No significant differences in mesozooplankton biomass between areas were found. On the other hand, clear time-trends could be demonstrated and related to salinity and temperature, with P.elongatus biomass mainly dependent on salinity and T.longicornis, Acartia spp. and cladocerans biomasses dependent, to a large extent, on thermal conditions. Decreasing salinities since the early 1980s due to a lack of major inflows of highly saline water from the North Sea and increased river run-off, both triggered by meteorological conditions, obviously caused a decrease in biomass of P.elongatus. Contrarily, the standing stocks of the other abundant copepod species and cladocerans followed, to a large degree, the temperature development and showed, in general, an increase. The shift in species composition during this period is considered to be a reason for decreasing growth rates of Baltic herring (Clupea harengus) since the early 1980s, and for sprat (Sprattus sprattus) since the early 1990s. Generally, it is suggested that low mesozooplankton biomasses in the 1990s were caused, at least partially, by amplified predation by clupeid fish stocks.

Description: Qualitative historical benthos data (1902–1912) were compared with recent data (1986) to find long-term trends in epifauna species composition in the southern North Sea that may be attributed to fishery-induced changes. In general, the frequency of occurrence of bivalve species declined, whereas scavenger and predator species (crustaceans, gastropods, and sea stars) were observed more frequently in 1986. We suggest that these shifts can be attributed not only to the physical fishery impact, but also to the additional potential food for scavenging and predator species provided by the large amounts of discards and moribund benthos. Our findings are put into the perspective of the general development of the demersal fishery in the southern North Sea. Despite the problems with the historical data set, the comparison presented may be the best illustration achievable of the changes in the benthos from a near-pristine situation to the present conditions after long-term disturbance.

Description: Oithona similis is an abundant but poorly studied cyclopoid copepod in the brackish Central Baltic Sea. We describe the spatio-temporal distribution of O. similis in a Central Baltic deep basin (Bornholm Basin) during spring and summer 1999. Using vertically resolving sampling in parallel with hydrographic measurements, we found the copepod to dwell in the permanent halocline characteristic of a Central Baltic deep basin. The habitat of O. similis is thus limited from above by low salinity and from below by low oxygen conditions, both characteristic for the area. Horizontally resolving sampling yielded abundance surfaces which were compared by analysis of variance showing similar patterns among sampling dates. Comparison with flow fields from a three-dimensional hydrodynamic model suggests that the horizontal distribution is primarily the result of circulation in the dwelling depth. The study shows how the physical environment in the area determines the spatial distribution which might affect abundance and production of this copepod.

Description: The seasonal development of bacteria was studied in the hypertrophic coastal lagoon Ciénaga Grande de Santa Marta (Caribbean coast of Colombia). This large but only 1.5 m deep lagoon is subject to strong seasonal variations of salinity from almost fully marine (April/May) to brackish conditions in October/November. Chlorophyll ranged from 6 to 182 μg L−1, and gross primary production amounted to 1690 g C m−2 per year. Total bacterial number (TBN) ranged from 6.5 to 90.5 × 109 cells L−1 and bacterial biomass (BBM) from 77 to 1542 μg C L−1, which are among the highest ever reported for natural coastal waters. Neither TBN nor BBM varied significantly with salinity, phytoplankton or seston concentrations. Only the bacterial mean cell volume showed a significant relation to salinity, being highest (0.066 μm3) during the period of increasing and lowest (0.032 μm3) during decreasing salinity. Bacterial protein accounted for 24% (19–26%) and phytoplankton protein for 57% (53–71%) of total seston protein. The ratio (annual mean) of bacterial carbon to phytoplankton carbon was 0.44 (range 0.04–1.43). At low phytoplankton abundance [chlorophyll a (Chl a) 〈 25 μg L−1], bacterial carbon was almost equal to phytoplankton biomass (i.e. the mean ratio was 1.04). In contrast, at Chl a 〉 100 μg L−1, BBM was low compared to phytoplankton biomass (the mean ratio was 0.16). In general, BBM varied less than phytoplankton biomass. Most probably, the missing correlation between bacterial and phytoplankton variables was due to (i) organic material partly derived from allochthonous sources serving as food resource for bacteria and (ii) a strong resuspension of bacteria from the sediment caused by frequent wind-induced mixing of the very shallow lagoon.

Description: Throughout the 1980s, reproductive success of the top-predator cod declined and stock sizes of the main prey species herring and especially sprat, important planktivorous predators in the system, increased substantially. Although the hydrographic conditions conducive for survival of early life stages improved during the 1990s, recruitment success of cod remained far below average. As clupeids have been identified as major predators on cod eggs and larvae in the Baltic, increased predation may be an important factor hampering stock recovery. Results from stomach content analysis of herring and sprat during the spawning season of 1988–1995 and ichthyoplankton surveys in the Bornholm Basin (the only important spawning area of cod in the Central Baltic in this period) allow a comparison of estimated consumption rates by the predator populations with standing stocks and production rates of cod eggs and larvae. Despite uncertainties in the estimation procedure, the findings confirm substantial predation on cod eggs by both clupeid species. Especially at the beginning of the cod-spawning season, characterized by low zooplankton availability, sprat consumed a considerable proportion of the eggs produced. In 1993, the relative importance of zooplankton as prey increased, while fish eggs were encountered more rarely. In contrast, predation by herring remained on the same level or even increased, especially late in the spawning season. The larval stage of cod is not substantially affected by predation owing to limited spatial overlap between prey and predator.

Description: In a mesocosm study, the appendicularian Oikopleura dioica bloomed after the reduction of copepod abundance, and in a second treatment showed a significantly negative correlation with copepod densities. Calculations, together with field data from the Baltic Sea, suggest that common calanoid copepods may control appendicularian population dynamics.

Description: Birds are the most conspicuous, wide-ranging, and easily studied organisms in the marine environment. They can be both predators and scavengers, and they can be harmed by and can benefit from fishing activities. The effects of fishing on birds may be direct or indirect. Most direct effects involve killing by fishing gear, although on a lesser scale some fishing activities also disturb birds. Net fisheries and hook fisheries have both had serious negative effects at the population level. Currently, a major negative impact comes from the by-catch of albatrosses and petrels in long-lines in the North Pacific and in the Southern Ocean. High seas drift nets have had, prior to the banning of their use, a considerable impact on seabirds in the northern Pacific, as have gillnets in south-west Greenland, eastern Canada, and elsewhere. Indirect effects mostly work through the alteration in food supplies. Many activities increase the food supply by providing large quantities of discarded fish and wastes, particularly those from large, demersal species that are inaccessible to seabirds, from fishing vessels to scavengers. Also, fishing has changed the structure of marine communities. Fishing activities have led to depletion of some fish species fed upon by seabirds, but may also lead to an increase in small fish prey by reducing numbers of larger fish that may compete with birds. Both direct and indirect effects are likely to have operated at the global population level on some species. Proving the scale of fisheries effects can be difficult because of confounding and interacting combinations with other anthropogenic effects (pollution, hunting, disturbance) and oceanographic factors. Effects of aquaculture have not been included in the review

Description: The development and application of a dilution method for measuring primary production in coastal waters is described.

Description: To test the effects of diatom production on larval fish growth and condition, laboratory experiments were performed with larval North Sea cod reared on different algal food chains. These food chains were based on cultures of (a) the diatoms Skeletonema costatum and Thalassiosira weissflogii; (b) the dinoflagellate Heterocapsa triquetra; (c) the flagellate Rhodomonas baltica; (d) a diet composed of both Skeletonema and Heterocapsa food chains (1:1), and (e) a starvation group. These algae were fed to cultures of adult Acartia tonsa. Copepod eggs were collected, hatched, and the N1 nauplii (200l−1) were fed to post-yolk-sac larval cod. Results indicate that larval growth rates are significantly influenced by the content of essential fatty acids of the algal food source: growth rates were positively correlated with the content of DHA (C22:6ω3) and negatively with EPA (C20:5ω3). The ratio of ω3/ω6 fatty acids in the algal source had no significant effect. The highest and lowest growth rates were observed in food chains based on H. triquetra and T. weissflogii, respectively (means for days 14–16 of 4.0 and −4.7). The mixed diatom/dinoflagellate diet resulted in intermediate growth rates and condition. Regressions of growth rates against EPA and DHA content indicated no inhibitory effect of diatom production on growth in larval cod.

Description: We used otolith microstructure analysis to reconstruct the growth histories of larval radiated shanny ( Ulvaria subbifurcata ) collected over a 2-week period in Trinity Bay, Newfoundland. A dynamic 3-dimensional, eddy-resolving circulation model of the region provided larval drift patterns, which were combined with measurements of temperature and zooplankton abundance to assess the environmental history of the larvae. The abundance of juvenile and adult capelin ( Mallotus villosus ), the dominant planktivorous fish in this area, was monitored using five hydroacoustic surveys. The goal was to determine whether environmental histories are helpful in explaining spatial and temporal differences in larval shanny growth, measured as cumulative distribution functions (CDF) of growth rates. We found evidence for a selective loss of slower growing individuals and recognized considerable spatial differences in the CDF of larval growth rates. Consistent patterns in capelin abundance suggested that faster growing survivors, sampled at the end of the 2-week period, developed in areas of low predator densities. A dome-shaped relationship between temperature and larval growth was observed, explaining a significant but small amount of the overall variability (14%). Effects of experienced prey concentrations on larval growth rates could not be demonstrated.