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

Abstract

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.

Spring bloom dynamics and zooplankton biomass response on the US Northeast Continental Shelf

“…Thus, emerging copepods would have been able to also take advantage of an early bloom. Indeed, Durbin et al (2003) demonstrated that when chlorophyll concentration was high in February, C. finmarchicus egg production rates were higher and more early stages copepodites were present. Thus, rather than differences between the timing of diapause exit and phytoplankton phenology, it may be variability between zooplankton population development and phytoplankton phenology that leads to the observed differences in bloom magnitude.…”

“…Distinct winter blooms are episodic events; however, overwintering chlorophyll levels can be highly variable across the Northeast Shelf ecosystem and zooplankton abundance in the Gulf of Maine can be influenced by these winter blooms (Durbin et al, 2003). Thus, a winter phytoplankton index was developed by computing the average chlorophyll concentration for each production unit over January and February.…”

“…Assessing how spring bloom dynamics relate to shifts in spring zooplankton biomass is a first essential step to uncovering the potential mechanism driving spring zooplankton variability in the region. However, from the outset we have to be circumspect about the role of the spring bloom since there is evidence to suggest that winter blooms in the Gulf of Maine may play an important role in shaping spring zooplankton populations by fostering extra generations of key species like of C. finmarchicus during winter (Durbin et al, 2003).…”

Sediment flux and recent paleoclimate in JordanBasin, Gulf of Maine

“…This event extended through January 2011, and is coincident with the high flux of CaCO 3 that extended from November through January. L. retroversa is commonly observed in Gulf of Maine plankton tows and displays increased abundance in the late fall-winter months (Bigelow, 1926;Redfield, 1939;Durbin et al, 2003).…”

“…Recent hydrographic studies have documented increased freshening in the coastal waters that flow southward past Nova Scotia and into the Gulf, and that such changes affect the entire ecosystem (Townsend et al 2010). Over decades these are related to the NAO because when nutrient-rich warm slope waters flood the basins during the NAO positive phase, the productivity is higher (Drinkwater et al 2003, Durbin et al, 2003. However recent freshening at high latitudes coupled with intense regional warming, has led to an overall decrease in phytoplankton and zooplankton abundance over the past decade despite the prevalence of positive NAO conditions (Pershing et al, 2005;Townsend et al 2010;Mills et al, 2013).…”