Selective retention of two dinoflagellates in a well-mixed estuarine embayment: the importance of diel vertical migration and surface avoidance

Abstract

Diel vertical migration patterns of the dinoflagellates Gonyaulax tamarensis and Heterocapsa triquetra were monitored in an estuarine embayment subject to localized blooms of both species. A concurrent study of tidal flushing using a dye tracer demonstrated an efficient, densitydriven mixing process that exchanged water within the embayment at a rate of approximately 0.5 d-'. Loss rates of the whole pond populations of G. tamarensis and H. trjquetra cells were smaller, ranging between 0.02 and 0.13 d-'. The cells were thus able to maintain a non-mixed distribution even under weakly stratified conditions. This selective retention of the 2 species relative to water exchange was due to the differential advection of surface and bottom waters through the inlet channel and the general avoidance of high irradiance surface layers by the dinoflagellates. Both species migrated to irradiances equivalent to 30% of summer sunlight when nutrients were presumably non-limiting, resulting in subsurface aggregations 1 to 2 m deep. Under nutrient-limited conditions, G. tamarensjsmigration was restricted to irradiance at or below 10 to 15% summer sunlight. Planozygotes (a life-cycle stage preceding cyst formation) migrated in a manner indistinguishable from the remainder of the nutrientlimited G. tamarensis population. The results help to explain the dominance of dinoflagellates in such embayments, the localization of their blooms, and the distribution of their resting cysts in the region.

Seasonal niche strategy of the bloom-forming dinoflagellate Heterocapsa triquetra

“…This nutrient-rich water moves laterally into the peripheral sub-estuaries forcing a limited 2 layer flow circulation with sharply defined salinity gradients (Lapennas 1980, Ustach et al 1986. H. triquetra is positively phototactic and can migrate in such a way as to exploit these flow regimes to maintain higher than expected population densities (Braruud & Pappas 1951, Pietrese & Van der Post 1967, Anderson & Stolzenbach 1985, Lindholm & Nummelin 1999 ) in these sub-estuaries. Higher dilution rates in the main channel of the estuary during the same bloom periods limit the corresponding H. triquetra cell densities to ~1 to 5 × 10 6 l -1 (Hobbie 1971).…”

Physiological and behavioral diagnostics of nitrogen limitation for the toxic dinoflagellate Alexandrium fundyense

“…To date many adaptations have been identified within natural phytoplankton communities, some behavioral and some physiological. In the former category, it has long been recognized that motility behavior of marine phytoplankton allows them to migrate through temperature gradients and exploit sources of sub-surface nutrients in the field (Eppley and Harrison 1975;Blasco 1978;Anderson and Stolzenbach 1985;Passow 1991) and the laboratory (Eppley et al 1968;Kamykowski and Zentara 1976;Cullen and Horrigan 1981;Cullen et al 1985;Rasmussen and Richardson 1989;Santos and Carreto 1992;). …”

Submarine groundwater discharge to a small estuary estimated from radon and salinity measurements and a box model

“…1). Salt Pond is roughly circular, with a surface area of 82 200 m 2 , a maximum depth of roughly nine meters and a mean depth of 3.4 m (Anderson and Stolzenbach, 1985). There is no surface runoff to the pond, however there is a storm drain that extends from below Highway 6, near the pond.…”

“…The pond surface area changed according to the bathymetry presented in Anderson and Stolzenbach (1985), extrapolating the relationship between area and tidal height to the high tide mark. Current speed was estimated from this flow estimate and the cross-sectional area of the channel, solely for the purpose of estimating the impact on gas exchange (this turns out not to be important, as will be shown later).…”

“…Salt Pond surface area varies with tidal height in the model, based on the bathymetric map of Anderson and Stolzenbach (1985), according to: Salt Pond area (m 2 )=838 th 2 −15 700 th+71 714, where th=tidal height (m) above low tide. The total volume of the pond is assumed to be 250 000 m 3 at low tide and 380 000 m 3 at high tide.…”