P. W. Froneman

Salp/krill interactions in the Southern Ocean:spatial segregation and implications for the carbon flux

Available data on the spatial distribution and feeding ecophysiology of Antarctic krill, Euphausia superba, and the tunicate, Salpa thompsoni, in the Southern Ocean are summarized in this study. Antarctic krill and salps generally display pronounced spatial segregation at all spatial scales. This appears to be the result of a clear biotopical separation of these key species in the Antarctic pelagic food web. Krill and salps are found in different water masses or water mass modifications, which are separated by primary or secondary frontal features. On the small-scale (o100 km), Antarctic krill and salps are usually restricted to the specific water parcels, or are well segregated vertically. Krill and salp grazing rates estimated using the in situ gut fluorescence technique are among the highest recorded in the Antarctic pelagic food web. Although krill and salps at times may remove the entire daily primary production, generally their grazing impact is moderate (p50% of primary production). The regional ecological consequences of years of high salp densities may be dramatic. If the warming trend, which is observed around the Antarctic Peninsula and in the Southern Ocean, continues, salps may become a more prominent player in the trophic structure of the Antarctic marine ecosystem. This likely would be coupled with a dramatic decrease in krill productivity, because of a parallel decrease in the spatial extension of the krill biotope. The high Antarctic regions, particularly the Marginal Ice Zone, have, however, effective physiological mechanisms that may provide protection against the salp invasion. r 2002 Elsevier Science Ltd. All rights reserved.

Zooplankton structure and grazing in the Atlantic sector of the Southern Ocean in late austral summer 1993 Part 2. Biochemical zonation

The composition, biomass, distribution and grazing impact of dominant components of the meso- and macrozooplankton community were investigated along a transect occupied in the Atlantic sector of the Southern Ocean during the second cruise of the South African Antarctic Ecosystem Study conducted in late austral summer (Jan.–Feb.) 1993. Total zooplankton abundance along the transect ranged from 6 to 161 ind. m−3, and biomass levels ranged from 4. 2 to 80.5 mg DWt m−3. Elevated abundances and biomass were recorded at stations occupied in the vicinity of the Marginal Ice Zone (MIZ), the Antarctic Polar Front (APF) and Subantarctic Front (SAF). Throughout the investigation mesozooplankton, comprising mainly copepods, dominated numerically and by biomass accounting for >80% of the total. South of the APF, the copepods Calanus propinquus, Calanoides acutus and Metridia gerlachei numerically dominated zooplankton counts, while north of the front the zooplankton community was dominated by the copepods Calanus simillimus, Rhincalanus gigas and Lucicutia ovalis. Grazing impact of the six most abundant zooplankton taxa, accounting for up to 90% of all zooplankton counted at each station, demonstrated large spatial variability ranging from 0.01 to 18% of the chlorophyll (chl-a) standing stock or up to 89% of the daily phytoplankton production (DPP). The highest grazing impact along the transect was recorded within the Polar Front Zone where zooplankton removed between 4 and 18% of the chl-a standing stock or between 53 and 89% of DPP daily. Outside this region, zooplankton grazing generally corresponded to <5% of the integrated chl-a or <20% of the daily DPP. The implications of the spatial differences in grazing impact on the efficiency of the biological pump are discussed.

Physical-biological coupling in the waters surrounding the Prince Edward Islands (Southern Ocean)

Abstract The results of a macro-scale oceanographic survey conducted in the upstream and downstream regions of the Prince Edward Islands in austral autumn (April/May) 1989 are presented. During the investigation, the Subantarctic Front, upstream of the islands, was shown to lie initially south at 46°38′S, while downstream, the front remained in a northern position of approximately 46°S. Surface expressions of the front show that the Subantarctic Front forms a zonal band, while the subsurface expressions (200 m) show a distinct meander in both regions. In the upstream region of the islands, the northern branch of the Antarctic Circumpolar Current, the Subantarctic Front, influenced by the shallow bathymetry, was deflected around the northern edge of the islands. Water masses in this region were shown to modify gradually from Subantarctic Surface Water (7°C, 33.75) to Antarctic Surface Water (5°C, 33.70) as the Polar Frontal Zone was crossed. Downstream of the islands a wake was formed resulting in the generation of broad, cross-frontal meanders. As a consequence, warm Subantarctic Surface Water from north of the Subantarctic Front was advected southwards across the Polar Frontal Zone, while cooler waters, which had been modified in the transitional band of the Polar Frontal Zone, were advected northwards. In the downstream region a warm eddy consisting of Subantarctic Surface Water was observed. Its generation is possibly due to baroclinic instabilities in the meandering wake. Zooplankton species composition and distribution patterns during the investigation were consistent with the prevailing oceanographic regime. Four distinct groupings of stations were identified by numerical analysis. These corresponded to stations found north of the Subantarctic Front, within the warm eddy, located in the Polar Frontal Zone, and those stations associated with the meander. The groupings were separated by the Subantarctic Front, which appears to represent an important biogeographic boundary to the distribution of warm-water zooplankton species. Warm eddies in the downstream region of the islands may represent an effective mechanism for transporting warm water species across the Subantarctic Front.

Predation impact of carnivorous macrozooplankton and micronekton in the Atlantic sector of the Southern Ocean

Abstract The composition, biomass, distribution and predation impact of carnivorous zooplankton were investigated along a transect from SANAE to Cape Town during the second cruise of the South African Antarctic Marine Ecosystem Study (SAAMES II) in January–February 1993. The carnivore component of the pelagic community consisted mainly of six groups: amphipods, euphausiids, decapods, fish, chaetognaths and jellyfish. Amongst these, euphausiids (mainly Thysanoessa macrura and Nematoscelis megalops ), chaetognaths ( Sagitta gazellae and Eukrohnia hamata ) and myctophids were the most prominent groups throughout the transect. Jellyfish were abundant within the Marginal Ice Zone (MIZ), while mesopelagic/interzonal decapods exhibited a peak in biomass north of the Subtropical Convergence (STC). Generally, carnivores comprised 10 to 30% of total zooplankton biomass. In frontal regions, such as the Antarctic Polar Front (APF) and the STC, their contribution to total zooplankton biomass decreased to ≤6%. Carnivores overwhelmingly dominated the zooplankton stock between the Subantarctic Front (SAF) and the STC, accounting for 42 to 96% of the total. Gut content analysis showed that all the species investigated are opportunistic predators, generally consuming the most abundant mesozooplanktonic groups. Chaetognaths and euphausiids exhibited the highest impact on secondary standing stock along the transect. Total daily predation impact varied considerably along the transect but generally accounted for x =12%) of the total stock, were found in the region between the SAF and the STC, in conjunction with the high densities of chaetognaths. Carnivores may potentially contribute a regional downward flux of faeces equivalent to ≤5% of the local mesozooplankton stock per day.

Trophic importance of the chaetognaths Eukrohnia hamata and Sagitta gazellae in the pelagic system of the Prince Edward Islands (Southern Ocean)

Abstract Trophodynamics and predation impact of the 2 dominant chaetognaths Eukrohnia hamata and Sagitta gazellae were investigated at 19 stations in the vicinity of the Prince Edward Islands and at a 24-h station occupied at the sub-Antarctic Front in late summer (April/May) 1996. During the entire investigation, the zooplankton assemblages were numerically dominated by copepods with densities ranging from 21 to 170 ind. m−3. Amongst the copepods, Clausocalanus brevipes, Metridia gerlachei and M. lucens dominated accounting for >90% of the total. Generally, chaetognaths were identified as the second most important group composing at times up to 30% (mean = 14.7%) of total zooplankton abundance. Of the two chaetognath species, E.␣hamata was generally numerically dominant. Gut content analysis showed that both chaetognath species are opportunistic predators generally feeding on the most abundant prey, copepods. No feeding patterns were evident during the 24-h station, suggesting that both species feed continuously. The feeding rates of E. hamata ranged from 0 to 0.50 prey ind. day−1 and between 0 and 0.90 prey ind. day−1 for S. gazellae. The maximum total predation impact of E. hamata was equivalent to 5.2% of the copepod standing stock or up to 103% of copepod production per day. For S. gazellae the predation impact was lower, reaching a level of 3.2% of the copepod standing stock or 63% of the daily copepod production. Chaetognaths can, therefore, be regarded as an important pelagic predator of the Prince Edward Islands subsystem.

Summer distribution of netphytoplankton in the Atlantic sector of the Southern Ocean

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.

Composition and spatial variability of macroplankton and micronekton within the Antarctic Polar Frontal Zone of the Indian Ocean during austral autumn 1997

Abstract Net sampling and continuous acoustic measurements within the Antarctic Polar Frontal Zone (APFZ) and in the vicinity of the Prince Edward Islands were conducted during austral autumn (April/May) 1997 to describe the composition and distribution of macrozooplankton and micronekton, and to investigate their relations to the prevailing oceanographic regime in the area. Two major circulation patterns associated with the Subantarctic (SAF) and Antarctic Polar (APF) Fronts existed in the oceanic environment surrounding the Prince Edward Islands, promoting high cross-frontal mixing both upstream and downstream of the islands. Average abundance and biomass of macroplankton/micronekton in the top 300-m layer were 21 ind. 1000 m−3 and 467 mg DW 1000 m−3, respectively. Pelagic crustaceans (euphausiids and amphipods), fish, chaetognaths and gelatinous zooplankton dominated numerically and by biomass. Continuous acoustic measurements displayed elevated pelagic biomass at the SAF and APF. Although four groupings of stations were identified using cluster analysis, a single macroplankton/micronekton community was recognized in the top 300-m layer throughout the offshore area of the APFZ. A modification of the APFZ community was observed within the inter-island region. Subantarctic species dominated zooplankton samples throughout the APFZ, although subtropical species were also well represented at stations occupied in the northern region of the APFZ. A biological response reflected in macroplankton community composition, resulting from an extensive cross-frontal mixing, was observed within the APFZ around the Prince Edward Islands.

Mutualism between the territorial intertidal limpet Patella longicosta and the crustose alga Ralfsia verrucosa

Mutualistic relations between plants and animals are well documented on land but have received less attention in marine systems. This study examined the relationship between the territorial intertidal limpet Patella longicosta and the crustose brown alga Ralfsia verrucosa. Adult Patella are found exclusively in association with Ralfsia, on which they feed, while Ralfsia occurs primarily, but not exclusively, in Patella territories. Ralfsia benefits directly from both the presence and the territorial behaviour of Patella. Algal productivity was assessed by measuring oxygen evolution and utilization in situ and deriving photosynthesis/irradiance curves. Productivity was increased by about 30% by the presence of Patella in both summer (Pmax of grazed algae 0.0098; ungrazed algae 0.0063 mg C · cm-2 · h-1) and winter (Pmax grazed algae 0.0081; ungrazed algae 0.0053 mg-2 · C · h-1). Algal growth rates were not significantly increased by the application of limpet mucus in the laboratory. We did not examine nutrient regeneration by the limpet, but the increase in photosynthetic rate may depend on the limpets grazing pattern which creates secondary sites for growth. Ralfsia also benefited from the territorial behaviour of Patella. The effects of different grazing regimes were investigated in different seasons by removing territorial limpets and either excluding all limpets using copper-based antifouling paint, or allowing access to non-territorial limpets (mostly P. oculus) using partial paint barriers. Exclusion of all limpets resulted in rapid overgrowth of Ralfsia plants by the foliose green alga Ulva sp.. Where non-territorial limpets had access to the plants overgrowth was reduced but Ralfsia plants were entirely removed by destructive grazing. Non-territorial grazers removed 90% of Ralfsia plants within 4 weeks in summer and 60% in winter. In control treatments P. longicosta prevented overgrowth by Ulva and actively excluded vagrant grazers, preventing overgrazing. Based on these findings, the association between the limpet and alga can be regarded as a nonobligate mutualism.

Residency Patterns and Migration Dynamics of Adult Bull Sharks (Carcharhinus leucas) on the East Coast of Southern Africa

Bull sharks (Carcharhinus leucas) are globally distributed top predators that play an important ecological role within coastal marine communities. However, little is known about the spatial and temporal scales of their habitat use and associated ecological role. In this study, we employed passive acoustic telemetry to investigate the residency patterns and migration dynamics of 18 adult bull sharks (195–283 cm total length) tagged in southern Mozambique for a period of between 10 and 22 months. The majority of sharks (n = 16) exhibited temporally and spatially variable residency patterns interspersed with migration events. Ten individuals undertook coastal migrations that ranged between 433 and 709 km (mean  = 533 km) with eight of these sharks returning to the study site. During migration, individuals exhibited rates of movement between 2 and 59 km.d−1 (mean  = 17.58 km.d−1) and were recorded travelling annual distances of between 450 and 3760 km (mean  = 1163 km). Migration towards lower latitudes primarily took place in austral spring and winter and there was a significant negative correlation between residency and mean monthly sea temperature at the study site. This suggested that seasonal change is the primary driver behind migration events but further investigation is required to assess how foraging and reproductive activity may influence residency patterns and migration. Results from this study highlight the need for further understanding of bull shark migration dynamics and suggest that effective conservation strategies for this vulnerable species necessitate the incorporation of congruent trans-boundary policies over large spatial scales.

Trophodynamics of selected mesozooplankton in the west-Indian sector of the Polar Frontal Zone, Southern Ocean

Mesozooplankton community structure and grazing impact were investigated at 15 stations in the west-Indian sector of the Polar Frontal Zone during the third dynamics of Eddie impacts on Marion’s ecosystem cruise, conducted during April 2004. An intense frontal feature, likely the convergence of the Sub-Antarctic and Antarctic Polar Fronts, was identified running in a north-eastward direction across the survey area. Total integrated chlorophyll-a (chl-a) biomass ranged from 4.15 mg m−2 to 22.81 mg m−2 and was dominated by picophytoplankton at all stations. Mesozooplankton abundances ranged from 163.84 ind m−2 to 2,478.08 ind m−2 and biomass between 6.70 mg Dwt. m−2 and 23.40 mg Dwt. m−2. The mesozooplankton community was dominated almost entirely by copepods, which contributed between 35% and 79% (mean=63%; SD=±12%) of the total numbers. The pteropoda, Limacina retroversa, contributed up to 30% (mean=10%; SD=± 8%) of the total numbers. Numerical analysis identified two distinct mesozooplankton communities separated by the intense frontal feature, namely the Antarctic and the Sub-Antarctic Zone Groups. Ingestion rates of the four numerically dominant copepod species (Calanus simillimus, Clausocalanus spp., Ctenocalanus spp. and Oithona similis) and the pteropod, L. retroversa, were estimated using the gut fluorescence technique. Total grazing impact ranged from 0.156 mg (pigm) m−2 to 2.958 mg (pigm) m−2 or between 1% and 29% of the available chl-a per day. The four copepods contributed approximately 36% of the total daily grazing impact, while the pteropod contributed to a mean of 64%, indicating that this zooplankton group may play an important role in the Southern Ocean carbon cycle. In general, the highest daily grazing impact was exhibited in the Antarctic Zone Group (mean=12% phytoplankton standing stock per day).