ALBERT

Description: Grazing experiments were conducted with natural mesozooplankton from Kiel Bight, Germany, using radioactive labelled phytoplankton cultures and seston size fractions. The results of experiments using phytoplankton cultures indicated that bivalve veligers performed highest clearance of particles within a size range of 4.7 to 6.3 µm, whereas optimum particle size for copepods was 15 µm. The results of experiments using labelled natural seston size fractions identified bivalve veligers and appendicularians as those responsible for the removal of particles within the smallest size class (〈2 µm). Seston size fractions larger than 5 µm were mainly cleared by copepods and nauplii. As particle size increased, the contribution of copepod clearance to total zooplankton clearance within size classes increased from 57% (〈5 µm size class) to more than 81% (30 to 100 µm size class). When the nauplii clearance rates were included, the total copepod clearance accounted for 90 to 97.6% of the total volume cleared of particles bigger than 10 µm. Despite low abundances of bivalve veligers and appendicularians in Kiel Bight at the time of the experiment, we calculated that approximately 10 and 8.5%, respectively, of the carbon ingested by total mesozooplankton was due to veliger and appendicularian grazing. The importance of bivalve veligers might be seen in their grazing on seston particles that escape predation by copepods and on the amount of energy that is therefore directed from the water column to the benthos when larvae settle.

Description: Spatial and temporal variability in environmental factors can exert major influences on survival and growth of living organisms. However, in many key areas of fisheries science (e.g. growth, survival and recruitment determination), environmental heterogeneity is usually ignored because of insufficient environmental or fisheries data or lack of evidence that such heterogeneity impacts response variables. For the eastern Baltic Sea (ICES Subdivisions 25 to 32), we evaluated spatial and temporal differences in conditions affecting the survival of cod Gadus morhua L. eggs at survival on four distinct spawning sites within the assessment area. We intercalibrated ways of quantifying the volume of water ('reproductive volume') at each site where salinity, oxygen and temperature conditions permitted successful egg development. We have developed and compared a time series (1952 to 1996) of reproductive volumes among the areas to identify spatial differences. The results of 2 independent volume-estimation methods are comparable, indicating that highly significant differences exist among the sites, and that the westernmost spawning ground, Bornholm Basin, has on average the highest reproductive volume and the lowest variability among the 4 sites. These findings may be useful in evaluating how spatial and temporal variability in environmental conditions affect egg hatching success and possibly recruitment in the Baltic stock.

Description: Throughout the last 2 decades a shift from a cod- to a sprat-dominated system occurred in the upper trophic levels of the Central Baltic Sea. This was caused by a decline in the cod stock, due to recruitment failure and high fishing intensity, resulting in a decrease in predation pressure on sprat. Concurrently with the lowest cod stock size on record, sprat reached biomass values of above 2 × 106 t in 1992, being relatively stable afterwards. Besides predation mortality through cod and in recent years also an increasing fishing pressure, cannibalism on eggs may be a compensatory process limiting the reproductive success of sprat and hence contributing to the population regulation in the Central Baltic. Based on sprat stomach sampling on 21 cruises between March 1988 and July 1996 cannibalism on sprat eggs was investigated in the Bornholm Basin, one of the main spawning areas of Central Baltic sprat. Using a model of gastric evacuation to estimate daily food intake rates and a Virtual Population/Extended Survivor Analysis for computing predator population sizes, egg cannibalism rates were estimated. These were compared to egg abundance data from ichthyoplankton surveys and to preliminary estimates of seasonal egg productions. The study revealed significant interannual differences in the intensity of sprat egg cannibalism with considerable predation in 1990 to 1992 (〉15% of the egg abundance during peak spawning and 〉60% of the seasonal production) and a reduction in most recent years (〈16% of the corresponding abundance and production). As a possible reason for these differences a combination of changes in the vertical overlap of predator/prey and variability in the food environment were identified. Shortcomings of the applied methods and the possible impact of cannibalism on the reproductive success and population development of sprat in the Central Baltic Sea are discussed.

Description: Nanoplankton and picoplankton abundance and community grazing on picoplankton were determined in summer and autumn at several stations in a productive coastal environment (Georges Bank, NW Atlantic Ocean) and in an oligotrophic oceanic ecosystem (Sargasso Sea). Ranges of heterotrophic nanoplankton (HNAN) abundance were 1.2 to 3.6 x 103 cells ml-1 on Georges Bank, and 2.2 to 6.8 x 102 cells ml-1 in the Sargasso Sea. Ranges of phototrophic nanoplankton (PNAN) abundance in these ecosystems were 1.9 to 6.0 x 103 and 1.3 to 4.7 x 102, respectively. Mixotrophic nanoplankton (MNAN), operationally defined here as chloroplast-bearing nanoplankton that ingested fluorescent tracers, comprised an average of 12 to 17% of PNAN in surface waters in both environments during August and October. Mixotrophs at specific stations constituted as much as 38% of total PNAN abundance on Georges Bank and 30% in the Sargasso Sea. Mixotrophs represented up to 39% of the total phagotrophic nanoplankton abundance (MNAN/[MNAN + HNAN]). Community grazing impact was estimated from the disappearance of fluorescent prey surrogates (fluorescently labeled bacteria, FLB; cyanobacteria, FLC; and 〈\3 µm algae, FLA). Absolute grazing rates (total picoplankton cells removed d-1) on Georges Bank exceeded those in the Sargasso Sea due to the greater abundances of predators and prey. However, there was overlap in the specific grazing losses at the 2 sites (ranges = 0.08 to 0.38 d-1 in the coastal ocean and 0.05 to 0.24 d-1 in the oligotrophic ocean). Rates of bacterivory were in approximate balance with rates of bacterial production (3H-thymidine uptake), but production exceeded bacterivory on Georges Bank during the summer cruise. These data are among the first documenting the impact of grazing on picoplankton in these environments, and they are consistent with the prediction that nanoplanktonic protists are major predators of picoplankton. While the proportion of phototrophs that are phagotrophic was highly variable, our study indicates that algal mixotrophy is widespread in the marine environment, occurring in both coastal and oligotrophic sites, and should be considered quantitatively in microbial food web investigations.

Description: This study tested whether the extreme scarcity of larger nanophytoplankton and microphytoplankton in the Gulf of Aqaba and in the open northern Red Sea is caused by nutrient limitation or by selective removal by grazers. Samples of near surface phytoplankton were incubated on board under a fully factorial combination of release from grazing pressure and release from nutrient stress. Release from grazing pressure by different size classes was obtained by sieving through 100, 20, and 10 µm size mesh screens. Release from nutrient stress was obtained by enrichment of Si alone and a full enrichment by N, P, Si and trace elements. Growth rates of most phytoplankton taxa showed a strong, positive response to the full nutrient enrichment and a weaker, but significant response to grazer exclusion. Several diatom taxa showed a weak positive response to Si enrichment. Thus, bottom-up control of medium-sized algae appears to be more important than top-down control.

Description: Increased nutrient loading favors macroalgal blooms in eutrophied coastal ecosystems. The main counteracting factor on this bottom-up support is top-down control by consumers. We asked (1) whether herbivore control on 2 bloom-forming macroalgae in the Baltic Sea varies between different algal life stages, (2) whether herbivores selectively feed on Enteromorpha spp. (Chlorophyceae) thereby supporting dominance of Pilayella littoralis (Phaeophyceae), and (3) whether various herbivore species differ in their effects. In comparative field and laboratory experiments, we analyzed herbivore pressure and selectivity on germling density and adult thalli of Enteromorpha spp. and P. littoralis. In the field, herbivores reduced macroalgal recruitment by 80% within 14 d indicating strong herbivore control at early life stages. Recruits of Enteromorpha spp. were significantly preferred over P. littoralis Adult thalli of both algae showed similar growth rates, but grazing rates were significantly higher on Enteromorpha spp. In laboratory experiments, Idotea chelipes (Isopoda), Littorina saxatilis (Gastropoda) and Gammarus locusta (Amphipoda) strongly reduced germling density, whereas effects of L. littorea were weak. I. chelipes and L. saxatilis significantly preferred germlings of Enteromorpha spp. over P. littoralis. I. chelipes had strong effects on adult Enteromorpha spp. but not on P. littoralis. The effects of G. locusta and L. littorea on both adult algae were smaller and not selective, and L. saxatilis did not feed on adults at all. Different herbivore feeding modes are discussed. We conclude that strong and selective herbivory on juvenile and adult Enteromorpha spp. favors dominance of P. littoralis in the Baltic. An effective control of macroalgal blooms in eutrophied areas can be supported by sustaining high herbivore density and species richness considering the variable and complementary effects of different herbivore species on different algal life stages.

Description: Experiments were carried out on Georges Bank, a productive coastal region in the northwestern sector of the North Atlantic Ocean, and in the oligotrophic western Sargasso Sea to examine the effects of nutrient (inorganic nitrogen and phosphorus) and organic carbon (glucose) additions on bacterial and phytoplankton growth. Four experiments were conducted in each environment. Phytoplankton growth was monitored over a 36 h period by following changes in the concentration of chlorophyll in unfiltered seawater and in seawater prefiltered through 5 μm screening to reduce grazing pressure. Bacterial production was estimated initially and after 24 h using the 3H-thymidine (TdR) method in unfiltered seawater and in 1 μm filtrate. Phytoplankton biomass increased significantly in response to nutrient additions in all but 1 experiment, whereas chlorophyll concentrations remained unchanged or decreased in all of the unamended (control) treatments or treatments supplemented with glucose. Responses of the phytoplankton community were similar for the 〈5 μm and unfiltered treatments. Bacterial production increased after 24 h in all of the treatments on Georges Bank, and there was little effect of nutrient or glucose addition in unfiltered seawater relative to unamended controls. However, glucose addition to the 〈1 μm filtrate caused substantial increases in bacterial production relative to controls and N/P-amended treatments in 2 of the experiments from this environment. Glucose had no stimulatory effect (relative to unamended treatments) in 3 of the 4 Sargasso Sea experiments, and only a marginal effect in the fourth. However, the addition of inorganic nitrogen and phosphorus in the latter ecosystem resulted in higher bacterial production (relative to unamended treatments or glucose addition) in 2 of the experiments with unfiltered seawater, and very large increases in 3 of the experiments with 1 μm filtrate. The magnitude of the changes in bacterial production differed greatly between unfiltered and filtered seawater in both ecosystems, indicating an important role for bacterial grazers in controlling bacterial population growth. The results of this study indicate different nutritional restraints on bacterial production in these contrasting environments.

Description: As one of the elements for a model on the food requirements of Humboldt penguins Spheniscus humboldti we determined, via gas respirometry, metabolic rates while swimming and resting in water. During rest in water at 19°C Humboldt penguins (mean body mass 3.6 kg) required 5.95 W kg-1. This corresponds to a thermal conductance in water of 0.2975 W (kg °C)-1 (at Ta 19°C and assuming a Tb of 39°C). When swimming in a 20 m long channel, metabolism rose from 8 W kg-1 at a speed of 0.6 ms-1 to 23.1 W kg-1 at 2.2 m s-1. Transport costs (the cost to move 1 kg of body mass over a distance of 1 m) reached a minimum at 1.4 ms-1 with 8.1 J (kg m)-1, which corresponds to 0.89 J (Nm)-1. We corrected for acceleration and deceleration in the channel to determine transport costs of free-ranging Humboldt penguins travelling at sea, which were calculated as 7 J (kg m)-1 (0.71 J [Nm]-1), at 1.7 m s-1. Birds feeding chicks need to balance the costs of either (1) returning to the breeding island for the night and travelling back to the feeding grounds in the morning or (2) incurring increased thermoregulatory costs associated with resting at sea overnight. Simple calculations show that at water temperatures of 19°C we expect Humboldt penguins to show a tendency to remain at sea overnight if foraging areas are 〉4 km from their island. In colder waters (12°C), this distance increases to 〉9 km. Using previously published data on at-sea activity of Humboldt penguins, we found that foraging costs during chick rearing amount to 340 g anchovies d-1. Finally, we present a general model to convert Humboldt penguin activity data at sea to food requirements.

Description: The large temporal and spatial variability in carbon isotope fractionation of marine phytoplankton (ε p) is thought to reflect differences in environmental conditions. Meaningful interpretation of this variability requires an understanding of the processes responsible for phytoplankton isotope fractionation. While numerous factors have been suggested to potentially influence ε p, recent theoretical and experimental evidence has emphasized the primary role of phytoplankton growth rate (µ) and CO2 concentration ([CO2aq]) in controlling ε p. Experimental examination of the relationship of ε p with µ and [CO2aq] in studies using different experimental approaches, however, has yielded inconsistent results. Here we directly compare new and previously published data on ε p as a function of CO2 concentration and growth rate for the marine diatom Phaeodactylum tricornutum. When grown under nitrogen-deficient conditions (nitrate-limited chemostat), ε p of P. tricornutum decreases with increasing growth rate. In contrast, under N-replete conditions ε p values are considerably lower at comparable growth rates and CO2 concentrations and are largely insensitive to a 3-fold increase in growth rate due to increasing photon flux density. In both experimental approaches, ε p shows a relatively small CO2 sensitivity in the range of CO2 concentrations naturally occurring in the ocean (8 to 25 µmol kg-1). Below ca 5 µmol CO2 kg-1, a strong decline in ε p with decreasing [CO2aq] is observed. The apparent difference in ε p responses between nitrate-limited and light-controlled cultures of P. tricornutum suggests a principal difference in carbon acquisition for different growth-rate-limiting resources. A mechanistic explanation is proposed and potential implications for the interpretation of phytoplankton carbon isotope fractionation are discussed.

Description: In the Baltic Sea, rye tested how short nutrient pulses of different lengths and frequencies affect macroalgae, epiphytes, grazers and their interactions. We hypothesized that even small-scale variations in nutrient supply may have significant impacts by favoring fast-growing epiphytes which can cause large-scale declines of canopy-forming macroalgae. In a factorial field experiment single plants of the canopy-forming macroalga Fucus vesiculosus with and without epiphytes were exposed to pulses of elevated nutrients (N and P) over 25 d. Five 1 h pulses given every 5 d had no significant effects. A single 5 h pulse increased the epiphyte load but not F. vesiculosus growth rate. In contrast, increasing epiphyte load caused F. vesiculosus growth rate to decline and attracted higher densities of gastropod grazers. These results indicate that a single nutrient pulse can have rapid direct and indirect effects on macroalgae and their associated epiphytes and grazers. Temporal variability of nutrient supply (five 1 h vs one 5 h pulse) plays a significant role in determining the response of primary producers and consumers to elevated nutrients.