Inter-annual Variability Of Phytoplankton Bloom And Its Ecological Effects In The Gulf Of Maine And Its Adjacent Coastal Waters

As one of the most important biological processes in coastal oceans, the phytoplankton bloom dynamics can affect the coupling of pelagic food chains, thus can have important ramifications for trophic interactions, overall system productivities and biogeochemical processes. Phytoplankton phenology has been one of the research hot-spots in the field of biological oceanography to understand the impact of climate change on marine ecosystem. In this thesis, Gulf of Maine and its adjacent coastal waters were selected as the study area. Remotely sensed ocean color data (SeaWiFS) and field observations analysis and numerical modeling have been used to study the spatial and inter-annual variability of both spring and fall phytoplankton blooms in the Gulf of Maine (GoM) and its adjacent coastal waters. The main environmental factors controlling the bloom timing and magnitude (peak timing and mean chlorophyll concentration during blooms were selected as the indices) have been identified, and the ecological effects of the climate-change-related bloom dynamics were also discussed.The ocean color data reveal a general pattern of westward progression of spring phytoplankton bloom (SPB), and an eastward progression of fall phytoplankton bloom (FPB) from the Nova Scotian Shelf (NSS) to GoM perspective. The spatial pattern of mean chlorophyll concentration in spring is similar to that in fall, with a lower concentration in the NSS and higher in the GoM. Inter-annually, there is a weak but significant tendency for years with earlier (delayed) SPBs to be followed by delayed (earlier) FPBs (r=-0.235, P<0.05), but the mean chlorophyll concentrations during SPBs are not correlated with that during FPBs. The inter-annual variability of SPB timing is significantly correlated with the sea surface salinity (SSS, r=0.415, P <0.01), but the FPB timing is correlated with both SSS and sea surface temperature (SST) (r=-0.28, P<0.05; r=-0.317, P<0.01). Further analysis shows that low SSS means a relatively stable water column and promotes the formation of stratification, which causes an earlier SPB and a delayed FPB. The correlation between SST and FPB timing is contrary to the theoretic mechanism. In reality, high (low) SST anomalies are often associated with high (low) SSS anomalies in this region. This is mainly due to variations in the advection of cold, fresh waters flowing into the study region.A 1-D ecosystem model, driven by surface heat and wind forcing and relaxed toward observed salinity profiles, was applied to simulate the inter-annual and decadal-scale variability of the phytoplankton blooms and plankton production from 1984-2007 in the Nova Scotian Shelf (NSS) and Gulf of Maine region (GoM). The model captured the mean observed timing and magnitude of the spring phytoplankton bloom (SPB) and fall phytoplankton bloom (FPB) in both systems, as well as observed inter-annual variations in SPB peak timing. Model simulations for both the GoM and NSS exhibited marked inter-annual variability in SPB and FPB timing (±2-3 weeks) and magnitude (up to～1 mg Chl m-3). Earlier SPBs and delayed FPBs are linked to enhanced water column stability generated by less saline surface water or sharper salinity gradients over the top 50m of the water column. The process-oriented numerical modeling experiments suggest that 1) salinity is the main factor influencing the bloom timing and magnitude in the NSS-GoM region, especially for the timing of SPBs; 2) compared to buoyancy forcing induced by vertical salinity gradient, the impact of surface heating and surface wind stress on the blooms variability is much weaker; and 3) the nutrient level controls bloom magnitude, but only has minor effect on bloom timing. Moreover, the modeled variation in annual primary productivity, mesozooplankton productivity, and particle export flux was modest (<10% of the mean). Years with high primary production were weakly associated with early SPBs (GoM: r=-0.205; NSS:r=-0.51), but there was no significant relationship with water column stability. Large phytoplankton is dominant in SPB, while small phytoplankton is dominant in FPB. Mesozooplankton biomass is the main part of zooplankton biomass in both SPB and FPB. Inter-annual variations in fisheries production due to changes in annual productivity are thus likely secondary to profound shifts in fisheries recruitment and production that have been linked to variations in SPB and FPB timing.