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 investigate metabolic activity in relation to seasonal food limitation (i.e. phytoplankton standing stock) at Adelaide Island (67°34¹S, 68°07¹W). Phytoplankton standing stock showed a brief but intense summer bloom and extended winter minima characteristic of Antarctic nearshore localities. Metabolic data were expressed for a standard sea urchin of 24.4 mm horizontal test diameter (this was the mean size of urchins used in the study). Two geographically close populations were studied, and strong seasonality was observed in MO2 at both sites. Mean austral winter minimal metabolic rates were 0.46 and 0.65 µmol O2 standard ind.-1 h-1 in 1997 and 1998 respectively at the main study site (North Cove). Mean winter oxygen-consumption rates at the second site (South Cove) were consistently lower throughout the study period (0.33 and 0.58 µmol O2 standard ind.-1 h-1 in 1997 and 1998 respectively), which coincided with reduced gonad mass in sea 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, AFDM) of a standard individual were also recorded mainly due to differences in reproductive condition, but also to seasonal accumulation and metabolic use 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 recorded was 1.44 and 1.62 µmol O2 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 ×2.5. Rapid metabolic increases occurred 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 summer seasons, contrasting 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 protein-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. However, even at the highest ratios (~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 employed by S. neumayeri does not exempt this species from the intense seasonal cycle of food availability (i.e. chlorophyll standing stock) at Antarctic nearshore locations.