Seasonal and interannual variability in temperature, chlorophyll and macronutrients in northern Marguerite Bay, Antarctica

We report data from the first 8 years of oceanographic monitoring in Ryder Bay, northern Marguerite Bay, Antarctica. These data form the oceanographic component of the Rothera Oceanographic and Biological Time-Series (RaTS) project. When weather and ice permit, the RaTS station is occupied every 5 days in summer and weekly in winter. Observations comprise a conductivity–temperature–depth (CTD) cast to 500 m and a water sample from 15 m, this being the depth of the chlorophyll maximum in most years. The water samples provide data on total chlorophyll (size-fractionated at 20, 5, 2 and 0.2 μm), macronutrients (N, P and Si) and dissolved organic carbon (DOC). The CTD profiles reveal strong seasonality in the topmost Antarctic Surface Water (AASW) driven by summer solar heating and winter cooling with brine rejection during ice formation. The depth of the winter mixed layer reaches a maximum in August, with annual maximum values ranging from ∼30 to >140 m. Below the AASW is the relatively aseasonal Winter Water (WW), and the bottom of the profile indicates the presence of modified Upper Circumpolar Deep Water (UCDW). Summer chlorophyll typically exceeds 20 mg m−3, with the peak in January. Vertical flux of phytodetritus is also predominantly in January. The summer bloom is dominated by large diatoms and colonial forms, whereas in winter most of the chlorophyll is in the nanophytoplankton (20–5 μm) fraction. Macronutrients show marked seasonality with N:P covariation close to Redfield (∼15.3) and Si:N stoichiometry ∼1.67. Summer DOC values show little seasonality and relatively high winter levels (>50 μM). Surface waters also exhibit a marked interannual variability, with ENSO as an important driver at subdecadal scales.