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Persistent dinoflagellate blooms in a small marine cove: II. Tidal fluxes of nutrients and phytoplankton
Furnas, M.J.; Smayda, T.J.; Tomas, C.R. (1990). Persistent dinoflagellate blooms in a small marine cove: II. Tidal fluxes of nutrients and phytoplankton. Mar. Nat. 3: 9-28
In: Marine Nature. Marine Science Institute, Yeungnam University: Kyongsan. ISSN 1225-990X

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  • Furnas, M.J.
  • Smayda, T.J.
  • Tomas, C.R.

    Pettaquamscutt Cove is unique among Rhode Island coastal waters for its persistent dinoflagellate blooms. A die! study of phytoplankton abundance, nutrients, temperature and salinity evaluated the hypothesis that circulation patterns provided a washout-reseeding mechanism which facilitates these dinoflagellate blooms. 22 samples were collected hourly over a 21 hr period covering two full ebb tide cycles and one-half full flood cycles. Die! freshwater inputs of inorganic nutrients from the major tributary were also examined. Observations supported the retention-reseeding hypothesis. About 50% of the population levels of the principal dinoflagellate species found then (Glenodinium uncatenum and Scrippsiella trochoidea) washed out of the inner basin (the site of the persistent blooms) on an ebb tide were reseeded into this basin on the ensuing flood tide. This physical mechanism supplements the potentially rapid growth rates reported for Glenodinium uncatenum and Gymnodinium simplex (another bloom species). These growth rates would appear adequate to compensate for expected tidally-induced, die! population washout rates based on Ketchum's model and cove flushing rates. Significant die! variations strongly characterized inorganic nutrient concentrations, influenced also by tidal inputs stage. Two contrasting trends occurred: inorganic nitrogen levels, particularly NH4, were higher in flooding waters; P04 and Si03 concentrations were higher in ebbing waters. Significant levels of N03 accreted into the inner basin from stream runoff were rapidly utilized during dinoflagellate growth and unavailable for tidal export to the outer basin. Two nitrogen pumps are thus operative within the inner basin of Pettaquamscutt Cove: freshwater delivery of N03 and flood tidal inputs of NH4 remineralized in the outer basin. These nitrogen enrichment processes further contribute to the persistence, retention and blooms of dinoflagellates within the inner basin, and reveal that these blooms are also regulated by nitrogen availability. Another nutrient pump operative within Pettaquamscutt Cove is the tidal export of P04 and Si03 from the inner to outer basin despite considerable utilization within the inner basin. Diatoms were surprisingly insignificant within the inner basin despite the nitrogen pump mechanism and high reserves of P04 and Si03-nutrient resources seemingly favorable to diatom growth in this shallow, well-mixed cove. This led to a relative underutilization of Si03 and its subsequent tidal export. Si03 concentrations were strongly correlated with salinity (r2=0.92) at the diel, tidal monitoring station located at the sill separating the inner and outer basins, and over which tidal exchange occurred. Si03 was therefore useful as a semi-conservative tracer of nutrient inputs and mixing. Tidal flux rates of the measured properties in the form of a general budget are presented. Available data on growth rates of some key dinoflagellate species, the circulation patterns and the nitrogen pump mechanisms provide reasonable explanations for the observed dinoflagellate blooms. An unresolved aspect of these unique blooms is the cause of the apparent repression of diatom growth. Possible explanations include: dinoflagellates outcompete the diatoms (=competitive exclusion); dinoflagellates secrete substances inhibitory to diatom growth(=allelochemic inhibition) and/or substances associated with freshwater runoff favor dinoflagellate growth over that of diatoms (=water quality effects). Comparisons are made between Pettaquamscutt Cove and other Rhode Island coastal lagoons, and further establish the locally unique aspects of this habitat.

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