Natural growth rates in Antarctic krill (Euphausia superba): II. Predictive models based on food, temperature, body length, sex, and maturity stage

We used the instantaneous growth rate method to determine the effects of food, temperature, krill length, sex, and maturity stage on in situ summer growth of krill across the southwest Atlantic sector of the Southern Ocean. The main aims were to examine the separate effects of each variable and to generate a predictive model of growth based on satellite-derivable environmental data. Both growth increments in length on moulting (GIs) and daily growth rates (DGRs, mm d-1) ranged greatly among the 59 swarms, from 0.58–15% and 0.013–0.32 mm d-1. However, all swarms maintained positive mean growth, even those in the low chlorophyll a (Chl a) zone of the central Scotia Sea. Among a suite of indices of food quantity and quality, large-scale monthly Chl a values from SeaWiFS predicted krill growth the best. Across our study area, the great contrast between bloom and nonbloom regions was a major factor driving variation in growth rates, obscuring more subtle effects of food quality. GIs and DGRs decreased with increasing krill length and decreased above a temperature optimum of 0.5°C. This probably reflects the onset of thermal stress at the northern limit of krill’s range. Thus, growth rates were fastest in the ice edge blooms of the southern Scotia Sea and not at South Georgia as previously suggested. This reflects both the smaller size of the krill and the colder water in the south being optimum for growth. Males tended to have higher GIs than females but longer intermoult periods, leading to similar DGRs between sexes. DGRs of equivalent-size krill tended to decrease with maturity stage, suggesting the progressive allocation of energy toward reproduction rather than somatic growth. Our maximum DGRs are higher than most literature values, equating to a 5.7% increase in mass per day. This value fits within a realistic energy budget, suggesting a maximum carbon ration of ~20% d-1. Over the whole Scotia Sea/South Georgia area, the gross turnover of krill biomass was ~1% d-1

The spring mesozooplankton community at South Georgia: a comparison of shelf and oceanic sites

Mesozooplankton (predominantly 200–2000 μm) were sampled at a shelf and an oceanic station close to South Georgia, South Atlantic, during austral spring (October/November) 1997. Onshelf zooplankton biomass was extremely high at 10–16 g dry mass m−2 (0–150 m), 70% comprising the small neritic clausocalaniid copepod Drepanopus forcipatus. Large calanoid species, principally Calanoides acutus and Rhincalanus gigas, contributed only 8–10%. At the oceanic station, biomass in the sampled water column (0–1000 m) was ∼6.5 g dry mass m−2 and 4–6 g dry mass m−2 in the top 200 m. Here, large calanoids composed 40–50% of the standing stock. Antarctic krill (Euphausia superba) occurred in low abundances at both stations. Vertical profiles obtained with a Longhurst Hardy Plankton Recorder indicated that populations of C. acutus and R. gigas, which overwinter at depth, had completed their spring ascent and were resident in surface waters. Dry mass, carbon and lipid values were lower than found in summer but were consistent with overwintered populations. Phytoplankton concentrations were considerably higher at the oceanic station (2–3 mg chlorophyll a m−3) and increased over the time on station. In response to this, egg production of both large calanoid species and growth rates of R. gigas approached those measured in summer. Onshelf phytoplankton concentrations were lower (<1 mg m−3), and low egg production rates suggested food limitation. Here phytoplankton rations equivalent to 6% zooplankton body C would have been sufficient to clear primary production whereas at the oceanic station daily carbon fixation was broadly equivalent to zooplankton carbon biomass

The deep-sea copepod fauna of the Southern Ocean: patterns and processes

In recent years, much attention has been paid to the Antarctic epipelagic fauna, as a result of the desire to increase our knowledge of ecosystem function and resource management. Unfortunately, our understanding of the polar pelagic deep-sea has not progressed as fast, and in common with many other parts of the world's deep ocean, knowledge is still fragmentary. As yet, we have an incomplete but evolving knowledge of species presence and distribution, but very little idea of how the extreme seasonality seen in the Southern Ocean might influence the deep-water fauna. An examination is made of species distribution and diversity, in relation to the latitudinal cline seen in many benthic groups, and the historical perspective offered by changing circulation patterns and sea temperature through geological time. Although a number of important frontal systems are found within the circumpolar Southern Ocean, the boundary is marked by the Sub-Tropical Convergence, which appears to be the major biogeographic boundary between it and surrounding provinces. Evidence for seasonality in various families is reviewed in light of what we know and can infer about their biology and particularly in respect of their bathymetric distribution, which in some groups appears to change with latitude

Moulting and growth of the early stages of two species of Antarctic calanoid copepod in relation to differences in food supply

Instantaneous measurements of moulting and growth of the early copepodite stages of 2 species of Antarctic copepod, Rhincalanus gigas and Calanoides acutus, were made at 4 regions around South Georgia during austral summer 1996/1997. Sea surface temperature was ~3°C across the study area whereas chlorophyll a concentrations were considerably higher towards the western end of the island. Despite this, moulting rate experiments showed that stage durations of both species were invariably short with no significant regional differences. Stage durations of R. gigas CI, CII and CIII averaged 9, 28 and 15 d respectively, and those of CII, CIII and CIV C. acutus were 4, 7, and 16 d respectively. Daily mass-specific growth rates were lower and less variable in R. gigas (mean 0.05 d-1) than in C. acutus (mean 0.14 d-1), and showed no measurable regional differences. Those for C. acutus however, were higher off-shelf at the western end of the island where the copepodites were heavier than elsewhere. In addition to variations in concentration of chlorophyll a, qualitative differences in the microplankton food supply may also have influenced growth rates. Large diatoms were far more abundant off-shelf at the western end of the island compared to elsewhere, where micro-flagellates and small diatoms dominated. It is suggested that the more opportunistic feeding mode of R. gigas gave stability to its growth rate, whereas C. acutus, which is predominantly herbivorous, was affected by the fluctuations in phytoplankton concentrations and species composition. Moulting occurred within a narrow range of carbon and dry mass for both species, although this range varied between stations

Response of the copepod community to a spring bloom in the Bellingshausen Sea

During Austral spring 1992, R.R.S. James Clark Ross worked a five station transect in the Bellingshausen Sea. The transect spanned unproductive waters under solid pack ice to an open-water bloom in the north, three weeks later. This paper addresses the ontogenetic development of the copepod community, and from grazing experiments on females of five species investigates their trophic response to a spring bloom. Copepods dominated the mesozooplankton in both numbers and biomass. Their mean biomass in the top 600 m was low (0.85–1.5 mg drymass m−3), which is similar to other high latitude oceanic localities in the Southern Ocean. Almost all the major copepod species underwent an ontogenetic seasonal ascent, from mainly below 250 m under the ice to the top 250 m at the open-water bloom stations. Based on the timings of migration, feeding and reproduction, the species appeared to fall into two broad categories. Firstly, the pronounced seasonal migrants, Calanoides acutus and Rhincalanus gigas, ascended from below 250 m into the top 50 m to coincide with the bloom. The few individuals of C. acutus in the surface layers prior to the bloom do not appear to have been feeding. Reproduction of R. gigas was later than that of C. acutus. Production of the second group, namely Calanus propinquus, Oithona spp. and possibly Metridia gerlachei, appeared to have been less keyed to the bloom. Their seasonal migration was less, and individuals were actively feeding prior to the bloom, albeit at rates about half of those measured during the bloom. Mass specific feeding rates of the species in this group were greater than those of C. acutus and R. gigas. In the top 250 m, carbon:nitrogen ratios of C. propinquus and M. gerlachei were less than those of R. gigas and C. acutus, which suggests less reliance on depot lipids at this time of year. Despite the cold temperatures, the mass-specific feeding rates of the five species measured were broadly comparable to summer values from more northern regions of the Southern Ocean. However, the estimated grazing impact of the copepod community at all the stations was negligible, rising to a maximum of only 8.4% of daily primary production at the most northerly bloom station. These low values result from the very low numbers of copepods in the epipelagic, particularly under the ice. Of the copepods measured, grazing was mainly by the adult female population of Oithona spp. before the bloom, and appeared to be mainly by Oithona spp. and C. acutus during the bloom

Distribution of pelagic larvae of benthic marine invertebrates in the Bellingshausen Sea

During November and December 1992, plankton samples were collected using a ring net of mesh size 200 μm vertically hauled through a 600 m water column, at five stations along a transect running north from the Allison Peninsula in the Bellingshausen Sea. Three stations were located over the continental shelf; two of these were ice bound, whilst the third was at the ice edge. Two other stations were in deeper, ice-free water. Sixteen different larval and juvenile types were found representing seven phyla: Echinodermata, Nemertea, Coelenterata, Mollusca, Annelida, Arthropoda and Bryozoa, of which the first two were the most abundant. Larval numbers and types decreased with distance offshore and away from permanent sea ice. The presence of many stages of nemertean larval development within a short time scale, in an area where developmental tends to be slow, suggests that reproduction occurs over an extended period and that the larvae have a long planktonic phase. The increased size of later developmental stages of the nemertean larvae indicates they obtain nutrition within the water column during winter, when little particulate food is present

A cyphonautes larva (Bryozoa: Gymnolaemata) from the Bellingshausen Sea, Antarctica

Bryozoans are an important component of the sessile, suspension-feeding fauna on the Antarctic continental shelf, (Knox & Cameron, 1970; Dell, 1972; Winston, 1983; Winston & Heimberg, 1988). Antarctic bryozoan faunas are systematically diverse, and display a broad range of colony morphotypes; they include fast-growing, short-lived species and long-lived perennials (Winston & Heimberg, 1988). Very little is presently known about the reproductive biology of Antarctic bryozoans, although it seems that a preponderance of species brood lecithotrophic larvae with a short, free-swimming phase. Planktotrophic larvae with a free-swimming phase of several weeks occur in a variety of gymnolaemate families, principally distributed in temperate coastal waters, and most often associated with specialized microhabitats or substrates. This type of larva, referred to as a cyphonautes (Zimmer & Woollacott, 1977), has not previously been reported from polar seas, and its occurrence in the Bellingshausen Sea, Antarctica, is thus of considerable interest

Rhincalanus gigas and Calanus simillimus: lipid storage patterns of two species of copepod in the seasonally ice-free zone of the Southern Ocean

The lipid and hydrocarbon composition of two species of Antarctic copepod, Rhincalanus gigas and Calanus simillimus, was investigated at two contrasting sites. Differences in the quantity of total lipid between sites were pronounced for R.gigas; females from a station near South Georgia where a bloom was in progress contained -8 times as much as those sampled in post-bloom waters in the Polar Frontal Zone (PFZ) some 450 km further north. In contrast, differences between sites for C.simillimus were less pronounced. The main lipid class for R.gigas was wax ester and for C.simillimus triacylglycerol. This fundamental difference is thought to reflect varying life-history patterns suggesting that C.simillimus may not undergo periods of pronounced food shortage. Pristane, a metabolite of phytol derived through the degradation of chlorophyll, was present in both species at both sites, indicating recent feeding activity, but the lack of the polyene diatom marker C21.6 in C.simillimus at the PFZ station suggested that it was largely feeding on other microp1ankton. Fatty acid analysis of C.simillimus offered further evidence that this species was omnivorous and that R.gigas was predominantly herbivorous 16:0 and 16:l generally accounted for slightly >50% of total fatty acids in both species; however, 16:0 was proportionately more abundant in C.simillimus and 16:l in R.gigas. The 16:1/16:0 ratio is usually >1 in diatoms, suggesting that the diet of C.simillimus contained items of prey other than diatoms, a fact confirmed in grazing experiments

Copepod growth and development around South Georgia: relationships with temperature, food and krill

Measurements of egg production rates (EPR) and growth of the early stages of 2 of the Southern Ocean biomass dominant copepods, Calanoides acutus and Rhincalanus gigas, were made over the course of 4 consecutive summer cruises which were carried out in the vicinity of South Georgia. For both species, EPR was found to be weakly but significantly related to chlorophyll a (chl a) although for C. acutus it was below maximum levels recorded in spring. Juvenile mass specificgrowth rates (g) were found to be body mass, stage and species dependent. Mean g for C. acutus stages CII to CIV decreased from 0.24 to 0.14, and for R. gigas stages CI to CIII from 0.06 to 0.04. Overall, values for both species were within the range predicted by recent global models of copepod growth. Neither stage duration nor g varied systematically with either temperature (mean, 0 to 60 m) or food (chl a, 0 to 60 m). However, carbon mass of nearly all species stages was negatively and significantly related to silicate levels (mol m–2, 0 to 60 m), suggesting the positive effect of past production levels. Ordination of zooplankton species occurrence by station across the survey area indicated that changes in abundance were more pronounced than changes in species composition, and that variation in total copepod abundance was also well explained by silicate levels. Our study indicated that changes in EPR, carbon mass and abundance of copepod populations at South Georgia were all strongly regulated by local primary production. Variation of chlorophyll biomass appeared largely dependent on temperature, rather than grazing pressure exerted by either copepods or krill. Krill at South Georgia were more abundant in colder, silicate replete waters and their presence is presumed to be governed by factors operating at the large scale. In contrast, copepod abundance appeared to differ in response to smaller scale variation in the environment and was linked through silicate to factors determining phytoplankton growth. In turn, chl a concentration was strongly and positively related to habitat temperature. This suggests the importance of the physical environment rather than grazing as ultimate factors controlling phytoplankton biomass in this productive ecosystem

Moulting rates of Calanus helgolandicus: an inter-comparison of experimental methods

An inter-comparison of three commonly reported methods for estimating the stage duration of copepods was made on Calanus helgolandicus from both wild and laboratory-reared populations. These three methods have not been compared previously in contemporaneous experiments. The methods were: (1) following the stage frequency of a laboratory-reared population over time, (2) sorting of individual species stages from the laboratory-reared population and from wild plankton samples, followed by incubation, and (3) sieve fractionating the wild plankton sample followed by incubation. We establish that estimates by these three methods made from the wild and laboratory-reared populations did not differ significantly. Handling of the animals did not affect estimates of moulting rate significantly, but there was a wide range in stage durations in identical copepodite stages which had experienced similar environmental conditions. We therefore suggest that the `Heinle' method would be most applicable to use on populations with a clear cohort structure in areas not subjected to high rates of advection, and which experience saturating food conditions. The sorted cohort method would be appropriate to areas which were subject to high rates of advection and to a population which had prolonged recruitment, but because this is a labour intensive method the sieved cohort method may be more appropriate when the plankton hauls are dominated by a particular species of copepod