We performed a multilevel factorial field experiment to identify the effects of five factors on sea urchins' (Strongylocentrotus droebachiensis) aggregating behavior. The factors were (1) source of urchins (kelpbed or barrens), (2) density of urchins (high or low), (3) location of treatment (kelpbed or barrens), (4) the presence and type of invertebrate predators (crabs or lobsters), and (5) season (summer or winter). These manipulative experiments were performed in flexible, resilient cages designed to withstand the severe wave surge in Nova Scotia's shallow subtidal environment. Interactions identified by ANOVA among the various factors showed that urchins aggregate more at high (20∙m−2) than at low (4∙m−2) density. The presence of lobsters in the kelpbed, and of crabs in the barrens, triggered the formation of even larger aggregations. These aggregations remain in the open even in the presence of predators. We argue that this behavioral mechanism (a defensive aggregation response to lobsters in the kelpbed) is the trigger that precipitates widespread destructive urchin grazing and the transformation of kelpbeds to barrens. Lobsters thus play two opposite roles. At low urchin density lobsters keep urchins in hiding and thereby contribute to kelpbed persistence. At higher urchin density, lobsters trigger the formation of large, exposed urchin aggregations that graze destructively on kelp. Urchin responses to predators are probably mediated by a combination of conditioning and sensitivity to biochemical cues. The large-scale change in community structure, from kelpbed to barrens, can thus be understood in terms of the adaptive behavioral responses of individual organisms. We review the roles of other predators in this system and show that they have varying effects on the intensity of urchin grazing, depending on urchin density, season, and habitat type.

In a submerged bed of eelgrass, newly formed leaves in winter and spring had their maximum levels of total organic matter (90% of dry weight), soluble organic fraction (45%), carbon (42%), and nitrogen (4.8%). These components all decreased as the leaves matured, aged, and died. Soon after death, a leaf had only 70% total organic matter, 28% soluble organic matter, 30% carbon, and 1.5% nitrogen. Intact dead leaves showed little further change in chemical composition. The commonly used crude protein determination (6.25 × nitrogen content) overestimated true protein by up to 180%. The carbon to nitrogen ratio (C:N) was an unreliable index of the nutritional value of the plant. Two growth forms were present, most probably in response to wave action and substrate composition. Daylength, not temperature, most probably controls the seasonal cycle of growth.

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Production of domoic acid (DA) by Pseudo-nitzschia multiseries (Hasle) was studied using continuous cultures with growth rdtcs ranging from 0 06 to 0.67 d-' At steady states, DA concentrations were 1.65 to 553.20 pg 1-' and production rates were 0 007 to 1.354 pg DA cell-' d-' Both were negatively correlated with rates of growth and silicate uptake. DA production was studled further by stopping the addition of fresh medium, thus producing batch mode experiments, in some of which silicate was allowed to decline, while in another silicate was increased to 85 pM. In those where silicate declined, DA production increased by a factor of 3. The maximum production rate attained was 3.17 pg DA cell-' d-' and the highest DA concentration in the culture was 768.5 1. 19 DA I-'. of which 664 pg 1. ' was in the cells (1 1.9 pg DA cell-'). In the experiment where silicate was enriched. DA production was suspended soon after the enrichment, but resumed when silicate in the medium became low. The results suggested differences in kinetics of DA production and growth under different supply rates of slhcate. There appear to be 2 types of conditions associated with DA production. When dissolved silicate is moderately low and there is a decline in overall physiological activity, intnnsic factors probably trigger the formation of a moderate amount of DA When dissolved silicate IS severely limiting, the extrinsic stress leads to considerably enhanced production of DA. 1989). During these blooms, silicate concentration in the sea water was low (<2 PM; Subba Rao et al. 1988), but the bloom population survived and produced the neurotoxin domoic acid.In batch culture, the production of domoic acid (DA) by Pseudo-nitzschia nlultiseries generally occurs in the stationary phase when cell division has stopped (Subba Rao et al. 1990, Bates et al. 1991 or when growth of the culture populations decline due to silicon limitation (Pan 1994). Pan et al. (1996 -this issue) proposed that DA product~on can be divided into 2 stages: the first coincides with the late exponential phase when growth proceeds slowly, and the second occurs when silicate in the medium is depleted and the culture is in the stationary phase. The rates of DA production during the second stage (13.67 to 30.20 fg DA cell-' d-l) were an order of magnitude higher than those during the first stage (0.97 to 4.98 fg DA cell-' d-l). This implies that (1) toxin production is not necessarily associated with complete cessation of cell division, and (2) this diatom produces more DA when cells are under severe silicate stress. 0

In a narrow, shallow estuary on the east coast of Canada, the dominant intertidal invertebrates were bivalve and gastropod molluscs. On a sand flat Mya arenaria produced 11.6 g∙m−2∙yr−1 flesh dry weight with a production:biomass ratio of 2.54, whereas Macoma balthica produced 1.93 g∙m−2∙yr−1 with a P:B ratio of 1.53. On a Spartina marsh, Littorina saxatilis produced 3.25 g∙m−2∙yr−1 with a P:B ratio of 4.11. Approximate P:B ratios were applied to biomass figures for four other species to give the following estimates of productivity: Mytilus edulis on Zostera beds 19.7 g∙m−2∙yr−1 flesh dry weight; M. edulis on Spartina beds 3.5 g∙m−2∙yr−1; Nassarius obsoletus 1.15 g∙m−2∙yr−1; Melampus lineatus 1.1 g∙m−2∙yr−1; Lacuna vincta 0.06 g∙m−2∙yr−1. A total production of the molluscs in the estuary is estimated at 4.7% of the production of Spartina and Zostera (all measured in kcal). It is postulated that molluscs are the chief primary consumers in the inlet.