Development of a deep chlorophyll maximum of Heterocapsa triquetra Ehrenb. at the entrance to the Gulf of Finland

Abstract

The development of a deep chlorophyll maximum (DCM) at a depth of 30-35 m was followed during a 15-d case study in July 1998 at the entrance to the Gulf of Finland. The study consisted of three 18-24-h periods of biological (chlorophyll a, phytoplankton, primary production), chemical (nitrate, phosphate) and physical (CTD, turbulence, vertical particle size distribution) measurements at an anchor station and six mesoscale towed CTD/ fluorometer mappings over the surrounding area. Exceptionally cold and windy weather led to a red tide of the dinoflagellate Heterocapsa triquetra instead of the cyanobacterial bloom that frequently occurs in late summer.Comparison of the estimated amount of nitrogen required for an H. triquetra bloom biomass with external loading affirmed that the bloom had been formed on the basis of the nitrate pool below the thermocline. The development of the bloom, therefore, led to the extremely deep nitracline. The DCM formed by H. triquetra developed at the top of the nitracline at an illumination of Ͻ0.1% of the sea surface illumination. A temperature-salinity analysis showed that the DCM was not caused by intrusions from inshore regions. It was concluded that the DCM was formed as a result of changing migratory behavior of H. triquetra after an upwelling event that fertilized the upper layer with phosphorus.

Seasonal niche strategy of the bloom-forming dinoflagellate Heterocapsa triquetra

“…These boundaries represent a rich source of inorganic nutrients and have stability properties favorable to H. triquetra accumulation. Kononen et al (2000) for example observed intense blooms of H. triquetra localized in 0.2 to 5 m thick layers 20 to 40 m below the surface of the water at the entrance to the Gulf of Finland in July 1998. Horizontal patch sizes were generally in the order of 1 km.…”

Bio-optical modelling of primary production in the SW Finnish coastal zone, Baltic Sea: fast repetition rate fluorometry in Case 2 waters

“…Motile warm-water species of small cryptophytes and larger dinoflagellates, specifically the deep-water-inhabiting Heterocapsa triquetra and Dinophysis spp. (Carpenter et al 1995, Kononen et al 2003 were subdominant at that time. The periods of lower salinity during the VABP and LSBP were mainly caused by fresh-water runoff from the semi-enclosed inland basin nearby.…”

Vertical migration of dinoflagellates: model analysis of strategies, growth, and vertical distribution patterns

“…This distribution pattern implies that dinoflagellate cells may perform asynchronous VM (AVM) with a period longer than 1 d. Moreover, the existence of persistent subsurface maxima of dinoflagellates near the pycnocline/nutricline has also been widely reported (e.g. Cullen & Horrigan 1981, Eppley et al 1984, Holligan et al 1984, Townsend et al 2001, Kononen et al 2003, indicating that some species of dinoflagellate simply stop migrating under certain environmental conditions (although active swimming is still required to maintain a particular depth without being dispersed by mixing). For the present discussion, subsurface aggregation is regarded as a special vertical migration pattern.…”