Sigrún Huld Jónasdóttir

Effects of food quality on the reproductive success of Acartia tonsa and Acartia hudsonica: laboratory observations

The effect of the chemical composition of food on the reproductive success of the copepods Acartia tonsa Dana and A. hudsonica Pinhey was studied in the laboratory. Laboratory-reared individuals were fed one of three monoalgal diets at different stages of growth: the diatom Thalassiosira weissflogii, the flagellate Rhodomonas lens and the dinoflagellte Prorocentrum minimum. The diet was analyzed for carbon, nitrogen, protein, carbohydrate and fatty acid content. Reproductive success was measured as eggs female-1 day-1 (Er) and as the hatching success of the eggs. The Er of Acartia spp. was correlated with protein and specific fatty acids [16:1ω7 (negative), 20:5ω3, 22:6ω3, and 18:0 (positive)] and, especially, the fatty acid composition of the algae expressed as the ω3:ω6 and 20:22 fatty acid ratios. The youngest diatom cultures and exponentially-growing flagellates displayed the highest Er; the lowest Er was recorded for females fed the senescent diatom cultures. The development time of eggs was affected by the age of the phytoplankton culture fed to the female. Hatching success of eggs decreased with the age of the algal culture, but no correlation was found with the meansured chemical components of the food.

Copepod recruitment and food composition: do diatoms affect hatching success?

Laboratory experiments were conducted to differentiate between factors controlling the hatching success of copepod eggs. Factors that could affect viability of eggs; viz food quality, female condition and external factors were investigated. In a series of experiments the copepod Acartia tonsa Dana was fed several different diets while egg production and hatching success were monitored. The diet was analysed for fatty acid content as an indicator of food quality. Both egg production and hatching were found to be affected by the nutritional quality of the food. Hatching was also highly dependent on female fertility. External effects were tested by exposing eggs to diatom extracts. Negative effects were only evident at high extract concentrations, but disappeared when aeration was supplied to the solution. Oxygen measurements showed that failure to hatch was due to hypoxia in the extracts. No inhibitory or toxic effects of diatom cell components on hatching could be found.

The trophic role of mesozooplankton at 47°N, 20°W during the North Atlantic Bloom Experiment

The biomass and grazing rates of three size classes of mesozooplankton—0.2–0.5 mm (small), 0.5–1.0 mm (medium) and 1.0–2.0 mm (large)—were quantified in the vicinity of 47°N, 20°W, from 25 April to 7 May (leg 4) and from 18 to 31 May 1989 (leg 5) as part of the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS). Biomass was inversely related to body size, with the small size fraction accounting for > 50% of the entire mesozooplankton biomass. Diel differences in biomass, however, were directly related to body size, indicating that vertical migration became more pronounced as the size of the animals increased. Total zooplankton biomass increased by almost a factor of 3 from the beginning to the end of the study. The average carbon-weight of individuals increased six-fold from leg 4 to leg 5 of the study. Carbon-specific rates of phytoplankton ingestion were (1) inversely related to body size; (2) greater at night for all size fractions; and (3) generally greater on leg 4 than on leg 5, particularly for the small size fraction. Grazing was dominated by the small size fraction (66% of the total grazing) on leg 4 and by the medium size fraction (44% of the total grazing) on leg 5. The removal of the daily primary production by mesozooplankton was not different from leg 4 to leg 5, averaging 2.7% day−1 (range 0.6–5.2% day−1). Comparisons of (1) estimated metabolic rates and (2) measured nitrogen excretion rates with daily rations of carbon and nitrogen, respectively, for zooplankton suggest that a phytoplankton diet only contributed about 50% of the daily carbon and nitrogen rations of animals. We hypothesize that mesozooplankton fecal pellets contributed < 5% of the POC flux out of the euphotic zone measured with particle traps. However, we estimate that during leg 5, the active flux of dissolved nitrogen out of the euphotic zone due to mesozooplankton diel vertical migration was 26% of the passive PON flux.

Perspectives on marine zooplankton lipids

We developed new perspectives to identify important questions and to propose approaches for future research on marine food web lipids. They were related to (i) structure and function of lipids, (ii) lipid changes during critical life phases, (iii) trophic marker lipids, and (iv) potential impact of climate change. The first addresses the role of lipids in membranes, storage lipids, and buoyancy with the following key question: How are the properties of membranes and deposits affected by the various types of lipids? The second deals with the importance of various types of lipids during reproduction, development, and resting phases and addresses the role of the different storage lipids during growth and dormancy. The third relates to trophic marker lipids, which are an important tool to follow lipid and energy transfer through the food web. The central question is how can fatty acids be used to identify and quantify food web relationships? With the fourth, hypotheses are presented on effects of global warmi...

Seasonal copepod lipid pump promotes carbon sequestration in the deep North Atlantic

Significance Every autumn across the North Atlantic, large numbers of zooplankton copepods migrate from the surface waters into the oceans interior to hibernate at depths of 600–1,400 m. Through this migration, they actively transport lipid carbon to below the permanent thermocline, where it is metabolized at a rate comparable to the carbon delivered by sinking detritus. This “lipid pump” has not been included in previous estimates of the deep-ocean carbon sequestration, which are based on either measurements of sinking fluxes of detritus, or estimates of new primary production. Unlike other components of the biological pump, the lipid pump does not strip the surface ocean of nutrients, and decouples carbon sequestration from nutrient replenishment, a process we term the “lipid shunt.” Estimates of carbon flux to the deep oceans are essential for our understanding of global carbon budgets. Sinking of detrital material (“biological pump”) is usually thought to be the main biological component of this flux. Here, we identify an additional biological mechanism, the seasonal “lipid pump,” which is highly efficient at sequestering carbon into the deep ocean. It involves the vertical transport and metabolism of carbon rich lipids by overwintering zooplankton. We show that one species, the copepod Calanus finmarchicus overwintering in the North Atlantic, sequesters an amount of carbon equivalent to the sinking flux of detrital material. The efficiency of the lipid pump derives from a near-complete decoupling between nutrient and carbon cycling—a “lipid shunt,” and its direct transport of carbon through the mesopelagic zone to below the permanent thermocline with very little attenuation. Inclusion of the lipid pump almost doubles the previous estimates of deep-ocean carbon sequestration by biological processes in the North Atlantic.

Effects of food concentration on egg-production rates of two species of Pseudocalanus: Laboratory observations

Experiments to determine the relationship between instantaneous egg-production rate and food concentration were carried out on Pseudocalanus newmani Frost and Pseudocalanus moultoni Frost from Puget Sound, Washington. The animals were fed a wide range of concentrations (0–300 μgC · 1−1) of the diatom Thalassiosira weissflogii (Grunow) Fryxell et Hasle. Despite similar morphology and size, P. newmani and P. moultoni appear to have intrinsically different responses in egg production to food concentration. P. newmani reached a maximum egg-production rate at low food concentrations (50 μgC·1−1) which can explain the observed year-round production of eggs of P. newmani in Puget Sound. In contrast, P. moultoni exhibited a more gradual increase in egg production with increasing food concentration which indicates P. moultoni is more sensitive to low food levels than is P. newmani. However, at high food concentrations, P. moultoni reached a significantly higher egg-production rate than did P. newmani. Only food concentration affected the egg-production rate of both species in these experiments as opposed to the size and condition of the animals.

Geographic Variation in Naupliar Growth and Survival in a Harpacticoid Copepod

Newly hatched nauplii of Scottolana canadensis (Willey) collected from two locales in Maine were larger than Maryland nauplii when females were reared under identical conditions (20[deg]C and high food concentration, 2.5 [times] 105 algal cells ml[minus]1). Under high food concentration, Maryland nauplii had faster growth rates (log10 [mu]m h[minus]1) than Maine nauplii, but survivorship was similar. Growth rates were lower under low food concentration (0.5 [times] 105 cells ml[minus]1), and were the same for all locales, whereas survivorship of the Maine nauplii through NV was higher than the Maryland nauplii. We hypothesize that size-related differences in naupliar feeding efficiency may explain the variation in survival under low food stress.