A high frequency time series at weathership M, Norwegian Sea, during the 1997 spring bloom: feeding of adult female Calanus finmarchicus

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110

We present observations of temperature, salinity and phytoplankton from late winter of 1999 obtained along a meridional transect running from 52 to 72°N through the northern North Atlantic and Norwegian Sea. The field work was conducted with the intention to find evidence for a postulated relationship between convection and primary production in the open ocean in winter. In the investigated area, convection ventilates water masses in a deep mixed layer which may cover several hundreds of meters, depending on atmospheric forcing and stratification. Inspired by the results of a coupled phytoplankton convection model, we postulated that sinking phytoplankton from the seasonal thermocline in autumn is dispersed within the mixed layer by vertical orbital motions and turbulence induced by convection. Orbital motions allow intermittent visits of plankton to the euphotic layer from much deeper depths. Hence, convection distributes the chance of plankton receiving light over the entire mixed layer. This supports production and allows compensation of losses due to sinking. We hypothesised that a winter stock of living phytoplankton in the open ocean would always coincide with the deep mixed layer formed by convection, irrespective of its depth. Indeed, along the entire meridional transect, living phytoplankton was found within the layer, whereas only marginal amounts or virtually no phytoplankton existed beneath it. The layer extent varied between 300 and 800 m. In the northern North Atlantic, average concentrations of chlorophyll a (as a measure of phytoplankton biomass) in the mixed layer were about 0.3 mg m -3, and 0.6 mg m -3 in the Norwegian Sea. We concluded that this winter stock forms the inoculum for a spring production in the open ocean. KEY WORDS: Primary production · Winter · Convection · Vertical orbital motionsResale or republication not permitted without written consent of the publisher

“…4) is very similar to the one measured during the spring bloom in May 1997 at Weathership M (i.e. 80 to 130 chl a mg m -2 ) (Irigoien et al 1998). …”

Section: Winter Versus Spring Biomasssupporting

confidence: 68%

“…An inspection of data obtained in early 1997 at Weathership M (Irigoien et al 1998, Dale et al 1999) revealed interesting features in time series of phytoplankton measured near the sea surface and at a depth of 100 m (Fig. 7), respectively.…”

Section: Transition From CML To Seasonal Thermoclinementioning

confidence: 94%

Convection and primary production in winter

Backhaus¹,

Hegseth²,

Wehde³

et al. 2003

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110

“…Although EP rates for C. finmarchicus were reduced during 2000, they were not reduced as greatly as may have been expected given the very low food levels (total chl a < 0.5 µg l -1 , > 5 µm chl a essentially zero), and our observations on GB during a post-spring bloom period when EP ceased while total chl a was around 0.5 µg l -1 (Campbell et al 2001a). A conclusion might be that EP by females, just after they emerge from diapause in winter and early spring, depends more on internal sources of energy than on external sources (Irigoien et al 1998). However, there is conflicting evidence on this issue at present.…”

Section: Discussionsupporting

confidence: 52%

“…In contrast, significant rates of EP by C. finmarchicus have been observed at pre-bloom chl a concentrations of only 0.2 to 0.4 µg l -1 (Niehoff et al 1999, Richardson et al 1999, this study). In 2 of these studies, measured (Irigoien et al 1998) and estimated (Richardson et al 1999) ingestion rates were insufficient to cover the energetic costs of EP. It is possible that in this species of Calanus, internal lipid stores provide most of the energetic requirements and some level of ingested material is required for gonad maturation and subsequent egg production (Ohman & Runge 1994, Niehoff et al 1999.…”

Section: Discussionmentioning

confidence: 99%

Interannual variation in phytoplankton blooms and zooplankton productivity and abundance in the Gulf of Maine during winter

Durbin¹,

Campbell²,

Casas³

et al. 2003

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An anomalous winter phytoplankton bloom was observed across the central Gulf of Maine during a winter cruise in late February 1999, but not during a similar mission to the same region in late February 2000. During the cruise in 1999, 0 to 40 m mean chl a concentrations were >2 µg l -1 , while in 2000 total chl a concentrations were <1.0, and > 5 µm size fractions were < 0.5 µg l -1. The presence of the bloom in 1999 could not be explained in terms of the critical depth hypothesis given conditions present at the time of the bloom. The large amount of colder, lower-salinity Scotian Shelf water present at the surface of the central Gulf of Maine in 1999 may have provided conditions suitable for the initiation of the bloom earlier during the winter. The presence of the winter bloom had a major effect on higher trophic levels. Zooplankton were >10 × more abundant in 1999 and included species typically not present in the central Gulf during winter. All developmental stages of planktonic copepods were present, including large numbers of older copepodid stages of species such as Calanus finmarchicus, indicating that the bloom had been proceeding for some time. RNA:DNA ratios of surface C. finmarchicus stage C5 and adult females were at or near maximum temperature-dependent values in 1999, but were considerably reduced in 2000. Egg production rates of C. finmarchicus were high in1999 but reduced in 2000. Both showed a curvilinear relationship with chl a. Analysis of gonads of females of other copepod species indicate similar between-year differences in reproductive rates. We suggest that the presence of the winter bloom in the central Gulf of Maine allows an extra generation of C. finmarchicus and other copepods to develop, leading to a buildup of the population during winter. This will have significant effects on downstream regions such as Georges Bank, which depend on re-supply from the Gulf of Maine.

“…nauplii on ciliates and diatoms, and avoiding the Phaeocystis sp. is possible (Irigoien et al 1998(Irigoien et al , 2000. In this situation, the mesozooplankton is actually removing the predators of small flagellates and picoplankton and diatoms, the possible competitors of Phaeocystis sp.…”

Section: Vertical Chlorophyll a Distributionmentioning

confidence: 99%

Freshwater control of onset and species composition of Greenland shelf spring bloom

Waniek¹,

Holliday²,

Davidson³

et al. 2005

. The phytoplankton community in the melting ice zone consisted of Phaeocystis sp., small flagellates (< 4 µm) and picoplankton, while diatoms were less abundant. Phaeocystis sp. contributed up to 15 g C m -2 to the carbon biomass (70% of total carbon measured), whereas the contribution of diatoms and flagellates to carbon biomass was relatively low, with up to 1.2 g C m -2 (5.7%) and up to 2.5 g C m -2 (11.7%), respectively. On the shelf the bloom starts at the very beginning of stabilisation (elevated N 2 values) which results solely from the release of meltwater. The locally restricted water stability leads to a patchy phytoplankton distribution in the Irminger Sea.