Seasonality of respiration and ammonium excretion in the Antarctic echinoid Sterechinus neumayeri

Oxygen consumption (MO2) and ammonium excretion rates were recorded monthly over 2 yr for the regular Antarctic echinoid Sterechinus neumayeri to inve stigate metabolic activity in relation to seasonal food limitation (i.e. ph ytoplankton standing stock) at Adelaide Island (67 degrees 34 'S, 68 degree s 07 'W), Phytoplankton standing stock showed a brief but intense summer bl oom and extended winter minima characteristic of Antarctic nearshore locali ties, Metabolic data were expressed for a standard sea urchin of 24.4 mm ho rizontal test diameter (this was the mean size of urchins used in the study ). Two geographically close populations were studied, and strong seasonalit y was observed in MO2 at both sites, Mean austral winter minimal metabolic rates were 0.46 and 0.65 mu mol O-2 standard ind.(-1) h(-1) in 1997 and 199 8 respectively at the main study site (North Cove), Mean winter oxygen-cons umption rates at the second site (South Cove) were consistently lower throu ghout the study period (0.33 and 0.58 mu mol O-2 standard ind.(-1) h(-1) in 1997 and 1998 respectively), which coincided with reduced gonad mass in se a urchins from this site, The minimal winter metabolic rates persisted for 7 and 6 mo respectively. Changes in the organic mass (ash-free dry mass, AF DM) of a standard individual were also recorded mainly due to differences i n reproductive condition, but also to seasonal accumulation and metabolic u se of body reserves, Energy liberated by loss of AFDM in the winter period equated to a metabolic use of 64 to 432 J mo(-1), whereas winter MO2 values indicated a metabolic use of 102 to 180 J mo(-1), Maximal summer MO2 recor ded was 1.44 and 1.62 mu mol O-2 standard ind.(-1) h(-1) in the 1997/98 and 1998/99 summers respectively, indicating a factorial increase over winter oxygen consumption rates of x3.1 and x2.5. Rapid metabolic increases occurr ed at the end of the winter, and were closely coincident with the onset of the phytoplankton bloom despite S, neumayeri being a deposit feeder. These MO2 rates are amongst the lowest reported for echinoids. Ammonium excretion followed a similar seasonal cycle, with peak rates of 262 and 415 nmol NH4 + standard ind.(-1) h-1 during the 1997/98 and 1998/99 suninier seasons, co ntrasting with winter minimum values of 81 and 72 nmol NH4+ standard ind.(- 1) h(-1) in 1997 and 1998. Echinoids are ammonotelic, and O:N ratios varied between 7 and 26 during the study period, Lowest values, indicating protei n-based metabolism, were found at the immediate start of the austral summer . O:N ratios gradually increased during the summer and following winter, as lipid and carbohydrate became more important in fuelling metabolism. Howev er, even at the highest ratios (similar to 25), protein still accounted for 50% of the metabolic substrate, The strong metabolic seasonality indicated by these data suggest that the generalistic deposit-feeding strategy emplo yed by S. neumayei does not exempt this species from the intense seasonal c ycle of food availability (i.e. chlorophyll standing stock) at Antarctic ne arshore locations.