ALBERT

Notes: Abstract The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by 15N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (K I), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high K I, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower K I). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, K Ivalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions.

Notes: Abstract Changes in the phytoplankton population caused by a wind-induced mesoscale eddy, first located off the Hawaiian chain on 29 August 1989, were examined from 6 to 12 September 1989. These cyclonic eddies produce upwelling of nutrient-rich water into the photic zone which may induce changes in the distribution of phytoplankton. The eddy affected the depth, magnitude and composition of the chlorophyll maximum. The eddy produced an upward shift in the distribution of phytoplankton, shoaling the chlorophyll maximum to 30 m. Pigment concentrations at the chlorophyll maximum were enriched up to 2.2 times inside the eddy. Outside the eddy, the upper mixed layer (UML, ≤50 m) was dominated by cyanobacteria whereas the deep chlorophyll maximum (DCM) was dominated by prochlorophytes, chlorophytes and chrysophytes. Inside the eddy, the UML showed an increase in deep water taxa but little change in cyanobacteria. The observed population changes probably resulted from increased growth rates and vertical transport of cells. Based on projections of the track of the eddy, we inferred that its trail created an area of enhanced chlorophyll that was not quickly consumed, and concluded that coupling between phytoplankton and grazers was weak. The presence of such cyclonic eddies may explain some of the variability in chlorophyll concentration found in the oligotrophic subtropical Pacific.

Notes: Abstract This paper describes a carbon budget for the spring phytoplankton bloom in Auke Bay, a subarctic bay in southeastern Alaska. The budget was constructed using semiweekly data on carbon production, particulate carbon in the water column, and cumulative sedimentation of carbon, chlorophyll a, and pheopigments. From these measured parameters, seasonal carbon consumption, utilization, and import/export terms were derived. The chlorophyll and pheopigment data were used to partition carbon sinking out of the photic zone between phytoplankton cells and fecal material. The difference between total carbon production and carbon available for consumption was attributed primarily to carbon import/export related to advection of water masses into and out of the bay. Separate budgets were developed for each of five sampling years (1985–1989). An average of 130±16 g C/m2 were produced by phytoplankton during each spring. Our model suggests that an average of 70% of this carbon was available for consumption by grazers within the bay; the remaining 30% is assumed to have been exported from the bay by advective transport. Of the available (non-exported) carbon, an average of 55% was consumed by grazers, 34% sank out of the photic zone in the form of uneaten algae, and about 11% remained at the end of the sampling period in the form of phytoplankton standing stocks. Overall, about 27% of the carbon produced each spring in Auke Bay (≈35 gC/m2) was used for growth and respiration by first-order consumers within the bay.

Notes: Abstract Continuous-culture results for Monochrysis lutheri grown on 12 h light-12 h dark cycles with a spectrum of ratios of nitrate and ammonium serving as limiting nutrients are compared with continuous light, exclusively nitrate, and exclusively ammonium-limited data for this species. The diel effects of the light regime on the maximum specific uptake rate are examined for both nitrate and ammonium. Synergistic effects on uptake by various initial concentrations of these two nutrients are presented. Preconditioning with light-dark cycles did not affect maximum uptake rate, but preconditioning on a combination of nitrate and ammonium gave much lower uptake rates than those observed for populations preconditioned on either nutrient exclusively. The implications of high maximum specific-uptake rates compared to maximum specific-growth rates in terms of the range of nitrate and ammonium ion concentrations associated with nutrient limitation are reviewed.

Notes: Abstract Data on phytoplankton primary production, biomass, and species composition were collected during a 5 yr (1985–1989) study of Auke Bay, Alaska. The data were used to examine the interannual differences in the timing, duration, and magnitude of the spring phytoplankton blooms during each year and to relate these differences to interannual variations in weather patterns. Within any given year, a pre-bloom phase was characterized by low available light, low rates of primary production, low biomass, and predominantly small (〈10µm) diatoms. During the primary bloom, integrated production rates rose to 4 to 4.5 g C m−2 d−1, and integrated biomass levels reached 415 to 972 mg chlorophyll m−2. Primary blooms were usually dominated by large diatoms (Thalassiosira spp.), and in a single year (1989) byChaetoceros spp. The primary blooms terminated upon nutrient depletion in the euphotic zone. Secondary blooms, triggered by nutrient resupply from below, occurred sporadically after the primary bloom and accounted for 4 to 31% of total spring production. The date of initiation and the duration of the primary bloom varied little from year to year (standard deviation 3 and 5 d, respectively). Seasonal production rates and biomass levels varied interannually by a factor of 2 to 3. In contrast, intra-annual variations of more than an order of magnitude, especially in biomass, occurred over periods as short as 10 d. These large variations over short time periods indicate the importance of synchronous timing between spring blooms and the production of larval fish and shellfish, which depend on an appropriate and adequate food supply for growth and survival. Parameters describing primary production (e.g. peak daily production, mean daily production, and total production during the primary bloom and the entire season) exhibited little interannual variation (coefficient of variation, CV = 10 to 19%), but a large degree of intra-annual variation (CV = 77 to 116%). Similarly, interannual variations in biomass (peak chlorophyll, mean chlorophyll) were also lower (CV = 20 to 33%) than intra-annual variations (CV = 85 to 120%).

Notes: Time taken by Pacific threadfin Polydactylus sexfilis juveniles to learn a conditioning reward was taken as the criterion for learning capability. Fish of 50 mm and 90 mm LF had a better learning capability than did smaller (22 mm, 36 mm) or larger (130 mm) size classes. Larger fish (90 mm and 130 mm) were quicker to recover from handling stress than were smaller fish. Based on these experiments it is suggested that 90 mm size class should have the highest adaptability when released into the wild.

Notes: The uptake and assimilation of nitrogen and carbon by shrimp were measured in 1200 L mesocosms using stable isotope enrichments. Labels were added via 15N-, 13C-glycine and amino acid mixtures in feeds or as 15NH4+ to pond water. Label was incorporated into shrimp via algal growth indicating that up to 31% of nitrogen requirements were derived from pond ecosystem dynamics. This value is low in comparison with other shrimp aquaculture isotopic tracer studies but is probably due to differences in shrimp-rearing conditions. Direct incorporation of the enriched feed label was low in shrimp muscle tissue (3.3% for 13C-glycine, 5.9% for 15N-glycine and 7.8% for 15N-amino acid mixture). Mass balance calculations indicate the remaining shrimp biomass was derived from feed, but loss of label into solution during feeding led to underestimation based on tracers. Incorporation of isotopic labels into feed as large molecular weight proteinaceous or microencapsulated/fat-coated compounds is recommended to prevent dissolution and loss.