Search

During the breeding season, seabirds must balance the changing demands of self- and off-spring provisioning with the constraints imposed by central-place foraging. Recently, it was shown that Northern Rockhopper Penguins at Tristan da Cunha in the South Atlantic Ocean switch diet from lower to higher trophic level prey throughout their breeding cycle. Here, we investigated if this switch is reflected in their foraging behaviour, using time-depth recorders to study the diving behaviour of 27 guard and 10 creche birds during the breeding season 2010 at Tristan da Cunha and obtaining complementary stomach contents of 20 birds. While no significant effects of breeding stage were detected on any foraging trip or dive parameters, stage/prey had a significant effect on feeding dive parameters, with dive duration, bottom time, and maximum depth explaining the majority of the dissimilarity amongst categories. We verified the previously shown dietary shift from zooplankton and cephalopods during the guard stage to a higher-energy fish-based diet during the creche stage, which was reflected in a change in dive behaviour from shorter, shallower to longer, deeper dives. This prey switching behaviour may reflect preferential selection to account for the increased physiological needs of chicks or simply mirror changes in local prey abundance. Nonetheless, we show that Northern Rockhopper Penguins demonstrate behavioural plasticity as a response to their changing energy requirements, which is a critical trait when living in a spatio-temporally heterogeneous environment. This ability is likely to be particularly important under extrinsic constraints such as long-term environmental change.

The use of indirect dietary markers, including stable isotopes, has immensely improved our knowledge of seabird trophic ecology throughout their annual cycle. Important aspects include differences in trophic niche between adults and chicks at the intra- and inter-specific levels and tissue-dependent differentiation in chicks. Using stable isotopic niche as a proxy for trophic ecology, we investigated how three closely related albatross species co-exist in the sub-Antarctic Prince Edward Islands. The effects of age, sex, tissue, and species on the isotopic niche were observed for Grey-headed Thalassarche chrysostoma, Sooty Phoebetria fusca, and Light-mantled Phoebetria palpebrata Albatrosses breeding on Marion Island. At the end of chick-rearing, carbon and nitrogen stable isotope values differed according to age, tissue, and species but not the sex of either adults or chicks. A complex pattern was revealed as the three species exhibited contrasting results. For example, values for δ13C or δ15N of chick blood could be depleted, enriched or similar relative to that of adults, depending on species. Stable isotope differences between blood and feathers likely reflect differences in their amino acid composition, while adult/chick differences will relate to their different physiological needs and diet. The results indicate that co-existence of the three species on the island is facilitated through resource partitioning among species in terms of foraging areas and in the trophic levels at which adults feed for themselves and their chicks. This work brings new insights into the effect of intrinsic factors on the foraging ecology of marine top predators.

The two fuliginous albatross species, Phoebetria fusca (the Sooty albatross, SA) and P. palpebrata (the Light-mantled albatross, LMA), are found breeding close to each other on the Prince Edward Islands (Southern Ocean). Adults of these two species are easy to identify in the field, but the chicks are difficult to distinguish on the basis of external morphology, especially at very young ages. Many ecological studies involve sampling of chicks as well as adults, and accurate field identification of chicks is thus crucial. Traditionally, the extent of the ring of pale feathers around the eye (eye-ring) has been used to identify the fledglings. The aim of our study was to investigate the utility of characters associated with the eye-ring (extent, measured as an angle, and colour) for the identification of Phoebetria chicks. This was achieved by comparison of identifications based on morphology, with positive identifications based on DNA sequence data from the mitochondrial cytochrome b gene. We confirm the value of morphological criteria in distinguishing LMA and SA chicks, and this technique appears to be accurate in the majority of cases (>80%). However, we recommend using molecular identification for borderline cases (angle of the eye-ring measuring between 85 and 95°) if independent confirmation of chick identity cannot be obtained (e.g. via observed feeding interaction with an adult).

Kelp beds are thought to provide a refuge for a variety of marine organisms by damping waves, changing hydrodynamic flow, offering substrata for epiphytic species, and altering the abundances of predators and prey. At the sub-Antarctic Prince Edward Islands, a typical flow-through system, kelp beds have the potential of acting as a prime habitat for a variety of pelagic species that would otherwise be swept away and lost from the island ecosystem. The current study investigates the effects of kelp beds on the abundance and composition of zooplankton and microphytoplankton, with the main aim of determining whether kelp beds act as a repository for larval stages in general, and for the swimming prawn, Nauticaris marionis, in particular. Salinity gradients created by freshwater run-off indicated that kelp beds at Marion Island retain surface water and restrict water exchange with the open ocean. Both the abundance and diversity of zooplankton were greatest within the kelp beds. This was partially due to the high abundance of kelp-associated species, but also to the apparent retention of naupliar larvae within the kelp canopy. Unlike the larvae of ostracods and copepods, caridean zoeae were rare, indicating that kelp beds are not a prime habitat for larvae of N. marionis. The larval retention mechanism for this species, therefore, remains unknown. Apart from retaining water and certain species of zooplankton, chlorophyll a/phaeopigment ratios confirmed the existence of a substantial detrital pool within the kelp beds. This retention of detritus may explain the observed importance of kelp-derived matter in the diet of many nearshore animals. In contrast to the zooplankton, kelp beds seemed to have little effect on the abundance and composition of microphytoplankton, and chlorophyll a values were low at all stations.

Sub-Antarctic bottom-dwelling caridean shrimps Nauticaris marionis were collected in April/May between 1984 and 1997 over the shelf region of the Prince Edward Islands (37 °50′E, 46 °45′S). N. marionis is a partially protandric hermaphrodite. Hatching probably occurs just before April each year, but may persist until May. During the 1st year N. marionis seem to survive in undetected localities, moult into juveniles, and then settle amongst the benthos from the plankton beginning probably after November. Diel vertical migration then occurs up to an unknown larger size. The vast majority of juveniles develop into males, most of which transmutate into females by April/May of their 3rd year. Reproduction can occur before all male secondary characteristics have been lost. A minority of individuals develop directly into females without passing through a male phase. Individuals older than 5 years are undetectable using samples of the sizes analysed, but they may well persist in the population. The von Bertalanffy growth parameters for N. marionis are tentatively identified as k=0.2353/year, L ∞=12.6 mm, t 0=−0.2828 years and WW ∞=2.03 g. Sex-reversal in N. marionis at Marion Island may be affected by the changing environment as sexual differentiation is probably determined during the planktonic stage.

Surface zooplankton and seabird densities and community composition in the Atlantic (between Cape Town and Sanae) and Pacific (between New Zealand and the Ross Sea) sectors of the Southern Ocean are described and related to oceanographic features. Samples were collected during two return voyages aboard the MV Benjamin Bowring as part of the Transglobe Expedition (1979–1981). High abundances of surface zooplankton and seabirds were consistently observed within the main frontal systems of the Southern Ocean. Generally, on a mesoscale significant correlations between surface temperature and the distribution of zooplankton or seabirds were observed. On a macroscale, the geographical positions of the zooplankton and seabird communities coincided with specific water masses. The results of this study suggest that appropriate food availability rather than water temperature is important for the determination of seabird distribution. The ecological importance of the recently described frontal zone associated with the northern boundary of the maximum winter expansion of sea ice is confirmed by biological data obtained in this study.

The relationships between three measurements of body size (carapace length, total body length and eye diameter) and three measurements of mass (wet, dry and ash-free dry weights) of the caridean shrimp Nauticaris marionis are presented. It is argued that a precisely defined carapace length is the most accurate indicator of body size. A conversion formula relating carapace length to total body length is also provided, as are conversion equations relating the different measures of mass.

The surface distribution of netphytoplankton (>20 μm) in the Atlantic sector of the Southern Ocean was investigated along two transects during early and late austral summer 1990/91. Sampling was under-taken at intervals of 60′ of latitude between 34° and 70°S for the analysis of nutrients and for the identification and enumeration of netphytoplankton. Peaks in total diatom abundances were recorded at the Antarctic Polar Front (APF), in the vicinity of the South Sand wich Islands, in the marginal ice zone and in the neritic waters of the Atlantic sector of Antarctica. Cluster analysis indicates the existence of two major zones between Southern Africa and Antarctica. Diatom abundance increased south of the Antarctic Polar From along both transects, which can be partially explained by gradients of silicate concentration. Small chain-forming species (e.g. Fragilariopsis kerguelensis and Nitzschia lineata) dominated the diatom assemblages in early summer, while larger species, such as Rhizosolenia hebetata f. semispina and Corethron criophilum, dominated late summer diatom assemblages. The predominance of typically ice-associated forms in early summer suggests that the release of epontic cells during ice melt provides the initial inoculum for the netphytoplankton biomass. These small diatoms are subsequently replaced by larger species.

A high resolution study of chlorophyll a and primary production distribution was carried out in the Atlantic sector of the Southern Ocean during the austral summer of 1990–91. Primary production (14C assimilation) and photosynthetic capacity levels at frontal systems were among the highest recorded during the cruise (2.8–6.3 mgC·m−3·h−1, and 1.3–4.7mgC·mgChl a−1·h−1, respectively). Blooms at ocean fronts were strongly dominated by specific size classes and species. This suggests that the increase in biomass was probably the result of an enhancement of in situ production by selected components of the phytoplankton assemblage, rather than accumulation of cells through hydrographic forces. This hypothesis is supported by the high variability of photosynthetic capacities at adjacent stations along the transects. Blooms (ca 2.7–3.5 mg Chl a·m−3) were found at three oceanic fronts (the Subtropical, Subantarctic and Antarctic Polar Fronts) during the early summer. These were equivalent to, or denser than, blooms in the Marginal Ice Zone and at the Continental Water Boundary. Seasonal effects on phytoplankton community structure were very marked. In early summer (December), netphyto-plankton (>20 μm) was consistently the major component of the frontal blooms, with the chain-forming diatoms Chaetoceros spp. and Nitzschia spp. dominating at the Subantarctic and Antarctic Polar Fronts, respectively. During late summer (February), nanophytoplankton (1–20 μm) usually dominated algal communities at the main frontal areas. Only at the Antarctic Polar Front did netphytoplankton dominate, with the diatom component consisting almost exclusively of Corethron criophilum. An early to late summer shift of maximum phytoplankton biomass from north to south of the Antarctic Polar Front was observed. Spatial covariance between silicate levels and water-column stability appeared to be the main factor controlling phytoplankton production at the Antarctic Polar Front. Low silicate concentrations may have limited diatom growth at the northern edge of the front, while a deep mixed layer depth reduced production at the southern edge of the front.

The recently described species Macrocystis laevis Hay is endemic to the Prince Edward Islands. Aerial photographs of Marion Island were used to outline the distribution of the kelp and to assess its cover. M. laevis occurs along the lee shore of the island, between the 5 and 20 m isobaths. Plant densities and gross plant morphology were measured by divers during April/May 1988. Net production was estimated from growth measurements taken in April/May 1988 and 1989 and again during August 1989. The mean biomass of kelp was 0.67 kgC·m−2 within the kelp beds. Net production was estimated at 7.7 gC·m−2·d−1 and 11.5 gC·m−2d−1 during the months of April and August respectively. M. laevis had a uniform frond-length frequency distribution, which suggests that only the oldest fronds are lost by wave action or senescence. Based on calculations for M. laevis and Durvillaea antarctica (the two species making up most of the macrophyte biomass) macrophytes are more productive per unit area than the phytoplankton but contribute less to the seas around the Prince Edward Islands by virtue of their small spatial coverage. Neither of the kelps lose much material as particulate or dissolved organic carbon through fragmentation. The extent of grazing on live M. laevis fronds is unknown, and only D. antarctica contributes to a macrofaunal detrital community. The contribution of M. laevis production to the nearshore ecology of the islands seems limited, as we suspect that almost all of its production is exported to the open ocean pelagic system.

The feeding dynamics of males and females of the sequential protandric shrimp Nauticaris marionis were investigated in late austral summer (April) 2002. Results of gut content analysis indicated that N. marionis is an opportunistic feeder preying on a variety of prey, with a preference for detritus, benthic amphipods and gastropods. Overall, pelagic prey made only an insignificant contribution to diet, and was only taken by males. Females of N. marionis exhibited cannibalism, particularly in incubation containers. There was no diel pattern in the feeding activity of N. marionis. In-situ daily rations of males (carapace length 7 mm) were equivalent to ≈10% and ≈5% of body dry weight, respectively.