Trophic status of subsurface oceanic and coastal waters surrounding South Orkney Islands: A view from planktonic protists

ALDER VIVIANA; FRANZOSI CLAUDIO; OLGUIN SALINAS H; MARSCHOFF ENRIQUE; ABBEDUTO M L; ROMBOLÁ EMILCE

Leuven

Simposio; XII SCAR BIOLOGY SYMPOSIUM; 2017

Scientific Committee on Antarctic Research (SCAR)

For the purpose of contributing to the understanding of productivity fluctuations within the first levels of the trophic web, the main goal of the study was to analyze the planktonic protist community structure at different spatial scales covering from oceanic to coastal waters around South Orkney Islands. A total of 36 samples were collected in February-March 2014 by means of Niskin bottles (10 m depth) in Scotia Bay (Laurie Island) and in the oceanic sector located at 60-64°S, 40-48°W (including the northern sector of the South Orkney Islands southern shelf MPA). Sample analyses included chlorophyll concentration and density of the main taxonomic groups of nano- and microplanktonic protists. Crustacean larvae were considered as well.Results for open waters revealed that average chlorophyll values were extremely low (0.14 µgChl L- 1 ). Flagellates (>5µm) contributed with 99% of total abundance (mean: 1.3 x 106 ind. L-1 ), followed by diatoms (5 x103 ind. L-1 ), dinoflagellates (3 x102 ind. L-1 ) and naked ciliates (2 x102 ind. L-1 ). Silicoflagellates, loricate ciliates and crustacean larvae reached densities comparatively lower. Highest abundances of flagellates (2.6 x 106 ) were found at isolated sites along the ocean-slope transition (62°S, 46°W) and in the proximity of Coronation Island (in coincidence with the sole chlorophyll peak found in the area: < 1µgChl L-1 ). Crustacean larvae showed up in 34% of the stations, mostly in the western sector; a single peak was detected in slope waters at the only site where dinoflagellates were absent. When excluding flagellate abundance from the analysis, a patch of simultaneous increase of other groups was observed in shelf waters SW of Laurie Island, resulting in the highest abundances of diatoms, dinoflagellates, silicoflagellates, and loricate ciliates within the area investigated. A comparison between open waters against the coastal waters of Scotia Bay revealed that average values of chlorophyll and diatom abundance were over twenty times lower in the former environment. Conversely, dinoflagellates and naked ciliates of open waters showed densities around 90 and 380 times higher, respectively. These findings suggest that while coastal waters show mesotrophic conditions in summer, the surrounding open waters rather resemble a typical oligotrophic status with average chlorophyll values even lower than those of Scotia Bay in winter. Relationships between each group and sea water temperature will be assessed.