No abstract

Diel feeding behavior and gut evacuation rates were investigated in adult female Acartia tonsa, in laboratory experiments at 20 "C using different concentrations of cultured Thalassiosira weissflogii as food, and in large mesocosms at -13°C using natural plankton as food. Diel feeding pattern was remarkably stable under the different food conditions and temperatures, maintaining about a 3-fold amplitude in gut contents and feeding rate over the diel period. The diel rhythm was retained even when the copepods were food limited and lost about 20 % body carbon during the course of the 24 h experiment. The persistence of the diel feeding rhythm in A. tonsa under starvation implies a strong selective pressure, possibly related to predator avoidance. Daily ingestion rates of copepods fed a high concentration of T. weissflogii at 20°C were about 90 000 cells d-l, or -148 % body carbon and 104 % body nitrogen, compared with 74 and 85 % body carbon and 40 and 45 % body nitrogen in mesocosm experiments at -13 "C with natural food. Daytime and night-time gut evacuation rates at 20 "C were not significantly different, indicating that the diel change in gut contents reflects a real change in feeding rate, rather than a change in the instantaneous gut clearance rate. Instantaneous gut evacuation rates were 0.0907 and 0.0894 min-l in 2 experiments at 20°C and 0.0415 and 0.0432 minp' at 12.7 and 13.3"C respectively. Day-night comparison of gut contents measured in individual A. tonsa, fed T. constricts at 8"C, revealed considerable individual variability. Range in gut contents was about 50-fold during the day (0.17 to 8.97 ng pigment copepod-'), and 100-fold at night (0.0 to 13.66 ng copepod-'). Distribution of gut content values was skewed towards lower values, especially during daytime. However all but 1 copepod in the day/night sequence had a measurable amount of food in the gut, indicating fairly recent feeding.

This study examines the growth and photosynthetic response of marine phytoplankton to a naturally occurring range of pH's. Growth rates were determined for Thalassiosira pseudonana and Thalassiosira oceanica via in vivo fluorescence measurements; photosynthetic rates were measured via I4carbon uptake using mesocosm tank assemblages of phytoplankton. A pH range of 7.0 to 9.4 was used for both sets of experiments and consistent declines of growth rate and photosynthesis were observed at high pH levels ( > p H 8.8). The pH response of the 2 phytoplankton species and the tank assemblages appeared to correlate with calculated concentrations of free carbon dioxide indicating a possible carbon substrate limitation at high pH. Half-aturation values for calculated [CO2] were determined for cell growth rates and for photosynthesis using the Monod equation (K, = 0.5 PM) and Michaelis-Menten equahon (K, = 1.3 PM), respectively. These values were within the range of values measured for phytoplankton in previous studes. Based on this evidence, it is suggested that at high pH levels the availabhty of CO2 may become limiting to manne phytoplankton growth and photosynthesis.

both regions was present as C5, with a small proportion of C4 and females There were vely few anlmals on GB. In the southern GOM about a third of the C. finmarchicus population was In the upper 80 m and actively feeding; the rerna~nder were below 120 m. On GB the C. fjnmarch~cus population was also feeding; gut p~glnent content in both areas was between 3 and 13 ng copepod-'. About one thlrd of the adult females on GB were reproductively a c t~v e ivh~le none in the GOM were reproductlvely active. By January C. finmarchicus C6 females formed a large proportion of the older stage a n~m a l s on the Bank and in surface waters of the southern GOM These were actively f e e d~n g and reproducing with a large cohort of young nauplii of C, finmarch~cus present in the southern GOM and smaller numbers on the northeast peak of the Bank. There were very few C. finmarchicus nauplii on the crest of the Bank or on the southern flank. Based on abundances and stage composition, it appeared that spawning began in late December and that repopulation of the Bank took place in the region of the NE peak, with nauplii and reproductlvely active females being transported onto this region of the Bank from the GOM.

Maximum ingestion rate (I,,,,,) in Acartja tonsa females from Narragansett Bay, Rhode Island. USA, when measured under standardized conditions of temperature (20 "C) and food (the diatom Thalassiosira weissflogii), varied by a factor of 2 to 3 (22 000 to 50320 cells copepod-' d-', 6.00 to 18.6 pg C copepod-' d-l, and 1.33 to 3.32 pg N copepod-' d-', or 121 to 376% final body C d-'. and 90 to 245% final body N d-l). Overall mean values were 38 200 cells copepod-' d -l , 10 3 pg C copepod-' d-', 1.96 pg N copepod-' d", 203% final body C d -' , and 146% final body N d ' Copepods gained weight during laboratory incubations, and consequently I,,,,, averaged 25.6 % and 19.9 O/o higher as a percentage of initial, than of final, body C and N. I,,,,, was most strongly related to the residual effects of field temperature, and secondarily to in situ food level, and initial body weight and condition factor (CF: weight per unit length). Weight and CF were strongly affected by the degree of food limitation. I,,, was highest in copepods with low initial body weight and CF, and from the poorest food conditions in the field. This compensatory increase in I,,,, resembles the hunger response described for other copepods, and would enable food-limited A. tonsa to more effectively exploit transient plankton blooms. Copepods increased significantly in both weight and CF during the 24 h laboratory incubat i o n~, demonstrating that body size was food limited even during plankton blooms. Mean weight increments over 24 h were 27.8% C, 20.3% N, and 22.6% dry weight. The amount of growth, and the growth efficiency, were inversely related to initial CF. Thus copepods that were most severely food limited in the field not only exhibited h~gher I,,,,, and higher growth rates, but also allotted a greater fract~on of ingested energy to growth, when provided with excess food in the laboratory.