The role of the euthecosome pteropod, Limacina retroversa, in the Polar Frontal Zone, Southern Ocean
Copepods, mainly Calanus simillimus, Oithona similis, Clausocalanus spp. and Ctenocalanus spp., typically dominated total mesozooplankton counts, accounting for, on average, between 75.5 % and 88.1 % (Mean = 77.4 %; SD = 13.4 %) of the total, during the present investigation. Results of the study indicate that L. retroversa may, at times, contribute substantially to total mesozooplankton abundances. During the study, L. retroversa contributed between 0.0 and 30.0 % (Mean = 5.3 %; SD = 7.1 %) to total mesozooplankton numbers. Significant small-scale variability in abundance and size structure of L. retroversa and abundance of copepods was minimal. Inter-annual variability, on the other hand, was significant between some years. Total pteropod numbers were greatest during April 2002 and 2004, while copepods exhibited greatest abundances during April 2004 only. Pearson’s Correlation analysis suggested that L. retroversa abundances were positively correlated to total surface chlorophyll-a (chl-a) concentrations. The significantly lower chl-a concentrations recorded during April 2005 may explain the reduced pteropod numbers observed during that survey.
The size class structure of L. retroversa comprised mainly small and mediumsized individuals during all four surveys. This corresponds well with records from the northern hemisphere (sub-Arctic and Arctic waters) where Limacina spp. are reported to exhibit maximum spawning during mid to late-summer. Higher abundances of large individuals only occurred during April 2005, when chl-a concentrations were very low; possibly the result of delayed spawning, due to reduced food availability.
Ingestion rates of the four most abundant copepods, determined using the gut fluorescence technique, ranged between 159.32 ng (pigm) ind[superscript -1] day[superscript -1] and 728.36 ng (pigm) ind[superscript -1] day[superscript -1] (Mean = 321.01 ng (pigm) ind[superscript -1] day[superscript -1]; SD = 173.91 ng (pigm) ind[superscript -1] day[superscript -1]). Ingestion rates of L. retroversa were much higher, ranging from an average of 4 28.68 ng (pigm) ind[superscript -1] day[superscript -1] in April 2002 to 4 196.88 ng (pigm) ind[superscript -1] day[superscript -1] in April 2005 (Mean = 4157.36 ng (pigm) ind[superscript -1] day[superscript -1]; SD = 35.37 ng (pigm) ind[superscript -1] day[superscript -1]). Average daily grazing rates for the pteropod varied between 0.39 mg (pigm) m[superscript -2] day[superscript -1] in April 2005 and 17.69 mg (pigm) m-2 day-1 in April 2004 (Mean = 6.13 mg (pigm) m[superscript -2] day[superscript -1]; SD = 11.04 mg (pigm) m[superscript -2] day[superscript -1]); corresponding average daily grazing impacts ranged between 8.4 % and 139.8 % of the phytoplankton standing stock in April 2005 and 2004, respectively (Mean = 48.5 %; SD = 84.5 %). Average daily grazing rates of the four copepods ranged from 4.58 mg (pigm) m[superscript -2] day[superscript -1] to 8.77 mg (pigm) m[superscript -2] day[superscript -1], during April 2002 and 2004, respectively (Mean = 6.28 mg (pigm) m[superscript -2] day[superscript -1]; SD = 5.94 mg (pigm) m[superscript -2] day[superscript -1]). Collectively, the copepods removed an average of between 31.6 % and 89.8 % of the phytoplankton standing stock per day, during April 2002 and 2004, respectively (Mean = 70.8 %; SD = 86.7 %). The daily grazing impact of the copepods accounted for an average of between 40.4 % and 87.8 % of the total zooplankton grazing impact, during April 2004 and 2005, respectively (Mean = 75.0 %; SD = 65.5 %). L. retroversa was responsible for an average of 52.4 % and 59.5 % of the total zooplankton grazing impact, during April 2002 and 2004, respectively. However, during April 2005, when L. retroversa numbers were significantly lower than previous years, the pteropod contributed an average of only 7.5 % to the total zooplankton grazing impact. Thus, during the present investigation,the pteropod was responsible for removing a mean of 48.9 % of the available phytoplankton (SD = 74.9 %).
The predation impact of the dominant carnivorous macrozooplankton and micronekton in the PFZ was determined during April 2004 and 2005 using daily ration estimates obtained from the literature. Additionally, gut content analysis was used to determine the contribution of L. retroversa to the diet of the dominant predators. Average predation impact ranged from 1.1 % and 5.7 % of the total mesozooplankton standing stock during April 2004 and 2005, respectively (Mean = 3.8 %; SD = 12.3 %). Chaetognaths and euphausiids dominated total carnivore numbers and made the greatest contributions to total predation impact during both years. Copepods appeared to be the main prey item of the dominant carnivorous macrozooplankton-micronekton in the region. L. retroversa was only detected in the gut contents of the amphipod, Themisto gaudichaudi, but not in either of the chaetognath species (Eukrohnia hamata and Sagitta gazellae) or the myctophid fish (Electrona spp.). The pteropod was found in 19 % of amphipod guts dissected. Pearson’s Correlation analyses showed that the four major predatory zooplankton groups found in the PFZ (chaetognaths, euphausiids, amphipods and myctophid fish) were positively correlated to abundances of L. retroversa, suggesting that the pteropod might be an important prey item for many of the carnivorous macrozooplankton/micronekton in the PFZ.
To conclude, L. retroversa may play an important role in the pelagic ecosystem of the PFZ, in austral autumn. However, ocean acidification and calcium carbonate undersaturation (as a result of increased anthropogenic carbon dioxide emissions), that is predicted to occur within the next 50 – 100 years, will most likely have significant implications for the Sub-Antarctic pelagic ecosystem if L. retroversa cannot adapt quickly enough to the changes.
School Location:South Africa
Source Type:Master's Thesis
Date of Publication:01/01/2007