The effect of nitrate-N availabil~ty on paralytic shellfish toxin production by the dinoflagellate Alexandrium tamarense was studied in a vertically stratified laboratory water column (tank) where swimming behavior could influence photosynthes~s and nutrition. Results were compared with those from batch and seml-cont~nuous cultures In w h~c h migratory behavior was not a factor The batch and semi-continuous cultures demonstrated a dlrect posltlve r e l a t~o n s h~p between N avallablllty and toxln content. Steady-state cultures, maintained at 2 contrasting rates of semi-continuous N supply, also demonstrated significantly diffel-ent cellular toxin profiles (relative proportion of toxlns) The tank experiment was carried out In a 2.1 m PVC cylinder (0.29 m internal d~a m e t e r ) and lasted for 24 d. lnitially, nitrate was replete throughout the water column (50 PM) and the h~g h l y toxic cells fol-med a thin surface layer which persisted throughout the 1 4 h light:lO h dark cycle. When nltrate was depleted in the surface layer a s a result of uptake by the phytoplankton, the cells began a nocturnal migration to the nitracline. During this phase the toxln content of the cells decreased gradually a s the C : N of the cells increased. In the third phase, the deep nitrate pool bvas exhausted and the cells penetrated deeper durlng the dark period. The toxln content of the crlls reached the lowest level during this phase. When nitrate was added to the deep layer, a fourth phase began, d u r~n g which nocturnal descent of the rnigratlng cells was again restricted to the nitracline; toxicity of the cells increased and C:N declined. Finally, N was added to the surface layer. During this flfth and final phase, cellular tox~city continued to rlse, C:N declined further, and the cells continued to mlgrate to the thermocline d u r~n g the dark period. The toxicity of the cells during the N-stratified phases of the water column exper~ment was ~ntermediate between the N-replete and N-depleted phases, indicating that A. tamarense is capable of producing PSP toxins from N acquired during a nocturnal descent. It is concluded that toxic dinoflagellates inhabiting N-depleted coastal waters are likely capable of sustaining growth and a moderate level of toxicity through nocturnal migrations to deep N pools.

The within-day, between-day and month-to-month variability of light attenuation and microalgal chlorophyll a (chl a ) and photosynthetic response was measured in San Antonio Bay, Texas, USA. Waters were shallow (<2 m) and turbid (attenuation coefficients of 0.7 to 15.3 m"), yet daily rates of primary production (0.1 to 2.5 g C d-') were comparable to those in much deeper and clearer estuaries. Chl a concentrations in the sediment (459 to 7837 mg m-3 in the surficial millimeter) were much higher than those In the water column ( 4 to 48 mg m-'). The benthic assemblages were photosynthetically competent, but dally benthic primary productivity was low (0.00 to 0.09 g C m-' d-'; an average of 2'Lb of productivity in the water colun~n) because of the very low irradiances at the sedimentwater interface. The high rates of production by the suspended mlci-oalgae were largely due to high chlorophyll-specific light-saturdted rates of photosynthesis. P:,:' 13.0 to 24.4 g C (g chl a)-' h -' ] , which were correlated positively wlth temperat~lre and inversely with the mean irradiance in the water column. The between-day and temperature-~ndependent variation in P$! was also correlated with F,,Chl-l, a n index of the proportion of functional photosystem 11 reaction centers. In turn, within-day variability in F,,Chl-l was inversely correlated with the mean irradiance in the water column in 8 of 10 observations, a pattern consistent with photoinhibition. A depression of P : ! ' caused by photoinhibition throughout the water column may therefore b e responsible for the inverse trend of P$' with mean irradiance in the water column. Short-term (h) variability in suspended chl a and turbidity was high (coefficient of variation = 13 to 7 5 % ) , but estimates of daily productivity could be predicted with reasonable fidelity (mean error 2 7 % ) from a single midday determination of chl a, the photosynthesis versus lrrad~a n c e response and the attenuation coefficient, along with daily incident radiation. The predictive power of a single observation was due to coherence in the variation of chl a, P: :' and the attenuation coefficient In the water column-the decrease in mean irradiance caused by resuspension was compensated for by concomitant Increases in Pc,hland suspended chl a Between-day variability in productivity of 15 to 5 2 % approached month-to-month differences, so the optimal use of resources in monitoring productivity would be to take slngle samples daily.

The vertical distributions of chlorophyll concentration and photosynthetic parameters of sediment-associated microalgae were measured with a resolution of 1 mm. Sediment and the overlying water were sampled at 3 sandy, shallow-water sites near Corpus Christi Bay, Texas, USA. All sediment samples had a floc in the surficial rmllimeter that contained as much as or more chlorophyll than the 0.2 to 0.6 m of water overlying them. The photosynthetic responses of the sediment-associated microalgae were comparable with those of the suspended phytoplankton. Below the surficial floc, constancy of chlorophyll concentration and photosynthetic-irradiance (PI) responses indicated the existence of a physically mixed layer in the underlying 8 to 15 mm. Photosynthetically competent algae were found below the mixed layer and well below the depth to which light penetrates the sediment (ca 1 mm). Primary production was more or less equally distributed between the surficial millimeter of benthos and the overlying water. The surficial floc and some of the underlying sediment may be readily resuspended so that 'benthic' microalgae can contribute significantly to both water column and benthic primary production.

1. To study further the influence of luminal nutrition on small bowel structure and function, segments of rat jejunum and ileum were completely excluded from intestinal continuity by Thiry-Vella by-pass operations. The effect of partial deprivation of luminal nutrition was also studied in jejunal segments that had been surgically transposed to a distal position in the intestinal tract.2. Macroscopically, by-passed jejunum and ileum both became narrowed and atrophic, whereas the intestine in continuity showed hypertrophic changes similar to those seen after intestinal resection.3. In by-passed intestine the pattern of villi changed from mucosal ridges to 'fingers' and 'leaves'. Although villous height and total mucosal thickness were both reduced, light microscopy showed that even 6 months after exclusion from intestinal continuity villous morphology was still retained.4. Epithelial-cell migration rate and turnover time were diminished in both jejunum and ileum after by-pass. In intestine in continuity the migration rate was increased, but the turnover time remained the same as in controls. 5. In by-passed jejunum, in vivo glucose absorption diminished progressively with time, but did not change in excluded ileal segments.6. After ileo-jejunal transposition, the jejunum showed no major morphological changes, but glucose absorption became significantly depressed.7. These results further support the concept that intraluminal nutrition exerts a major influence on cell turnover, villous morphology and absorptive function in the small intestine.

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