Akashiwo sanguinea is a cosmopolitan dinoflagellate that has been observed to form major blooms in coastal ecosystems around the world. A. sanguinea plays a major role in the ecology of many marine environments, including coastal ecosystems with variable salinities, where its euryhaline character makes it competitive. The study involved monthly water sampling and measurement of physical-chemical parameters at four sites from the upper reaches of the Caloosahatchee Estuary to San Carlos Bay bordering the Gulf of Mexico. Major A. sanguinea blooms were observed in the Caloosahatchee estuary, with biovolume up to 30.5×106 μm3 mL-1, equivalent to 740 cells mL-1, in the upper estuary. The latter regions of the estuary also had the greatest range of salinity and the highest total nitrogen and total phosphorus concentrations. The dynamics of blooms are discussed within the context of spatial and temporal patterns of salinity, temperature and nutrient concentrations, as they relate to hydrologic and climatic conditions.
We investigated the relationship between digestive function and ecological distribution for four related species of Gastropoda (Cerithidea rhizophorarum, Cerithidea cingulata, Batillaria multiformis, and Batillaria attramentaria) inhabiting the Tanaka River estuary (Mie Prefecture, Japan). We compared the digestive enzyme activities for four hard degradable carbohydrates, namely, cellulose, mannan, xylan, and laminarin. Among the investigated four species, Cerithidea rhizophorarum showed the highest cellulase, mannanase, xylanase, and laminarinase activities, and was also dominantly distributed in the estuary reed bed. The results of CN stable isotopic analysis suggested that this species assimilated organic matter derived from reeds. Hard degradable carbohydrates, including plant components derived from the reed bed or from dry land, accumulate in the sediment of estuaries. Estuarine benthic animals are assumed to consume these accumulated hard degradable carbohydrates, as well as microphytobenthos and particulate organic matter (including phytoplankton). Our present findings suggest that Cerithidea rhizophorarum is dominant in reed beds because it can more efficiently digest plant-derived carbohydrates than can Cerithidea cingulata, B. multiformis, and B. attramentaria. To the best of our knowledge, ours is the first study to demonstrate that the specific ecological distribution of related animal species with similar ecological traits can be explained by the efficiency of their digestive enzyme activity.
Species compositions of planktonic cells and benthic resting-stage cells of the marine diatom Skeletonema were investigated in Osaka and Tokyo Bays. Seven to 11 strains were isolated from each of the monthly water samples for species identification by molecular analysis. The species frequency of the strains indicated that S. sp. cf. marinoi-dohrnii complex was dominant throughout the year in Osaka Bay, whereas in Tokyo Bay the dominant species was replaced by S. japonicum during the cold season. The other species identified were S. costatum, S. japonicum, S. pseudocostatum, and S. tropicum in Osaka Bay, and S. menzelii and S. tropicum in Tokyo Bay. For species identification of resting-stage cells, sediments in each bay were sampled at 2–3 stations and stored in the dark for ≥85 days. Eight to 12 strains were isolated from each of the diluted sediments incubated at five temperatures (10–30°C). S. costatum, S. japonicum and S. sp. cf. marinoi-dohrnii complex were identified in these stains; the resting stage of S. japonicum was newly recognized. The dominant species of resting-stage cells in each bay largely agreed with those of planktonic cells on a yearly basis. S. sp. cf. marinoi-dohrnii complex in the sediment could germinate at all the temperatures but S. japonicum did not occur at 30°C. The results of seasonal occurrences and incubation experiments indicate that S. sp. cf. marinoi-dohrnii complex is eurythermal and S. japonicum prefers lower temperatures. Possible factors for the difference in the community of Skeletonema between Osaka and Tokyo Bays are discussed.
Studies on seasonal and spatial variations in the community structure of chaetognaths are limited despite their importance in marine plankton ecosystems. We examined community structures of chaetognaths in one-year monthly samples collected at three stations from inshore (Stn 1) to the shelf edge (Stn 3) in Tosa Bay on the Kuroshio coast of Japan. The yearly mean density of chaetognaths was 22.2 ind. m-3, consisting of Zonosagitta nagae (50.0%), Flaccisagitta enflata (17.4%), Serratosagitta pacifica (9.4%), and Aidanosagitta regularis (9.3%). The monthly dominant species were Z. nagae from March to July, F. enflata and A. reguralis from August to October, and F. enflata and S. pacifica from November to February. The densities of Z. nagae and F. enflata in the top 50 m were generally higher at Stn 1. In the 200-m water column at Stn 3, the densities of the dominant species were generally highest in the 0–25 or 25–50 m layer, and A. regularis was generally distributed in the shallowest layer, followed by F. enflata, Z. nagae and S. pacifica according to their population mean depths. The present results on the community structure and vertical distribution agree well with those in bays located further downstream the Kuroshio Current, indicating that the results are general features in temperate Kuroshio coastal waters. Comparison of chaetognath community structures with those in adjacent regions indicates that the dominance of Z. nagae in regard to the yearly mean abundance is characteristic to temperate Kuroshio Current coasts.