Phytoplankton production patterns in Massachusetts Bay and the absence of the 1998 winter-spring bloom

The seasonal productivity cycle and factors controlling annual variation in the timing and magnitude of the winter-spring bloom were examined for several locations (range: 42 degrees 20.35 ' -42 degrees 26.63 'N; 77 degrees 44.19 '- 70 degrees 56.52 'W) in Boston Harbor and Massachusetts Bay, USA, from 1995 to 1999, and compared with earlier published data (1992-1994), Primary productivity (mg C m(-2) day(-1)) in Massachusetts Bay from 1995 to 1999 was generally characterized by a well-developed winter-spring bloom of several weeks duration, high but variable production during the summer, and a prominent fall bloom. The bulk of production (mg C m(-3) day(-1)) typically occurred in the upper 15 m of the water column. At a nearby Boston Harbor station a gradual pattern of increasing areal production from winter through summer was more typical, with the bulk of production restricted to the upper 5 m. Annual productivity in Massachusetts Bay and Boston Harbor ranged from a low of 160 g C m(-2) year(-1) to a high of 787 g C m(-2) year(-1) from 1992 to 1999. Mean annual productivity was higher (mean- = 525 g C m(-2) year(-1)) and more variable near the harbor entrance than in western Massachusetts Bay. At the harbor station productivity varied more than 3.5-fold (CV = 40%) over an 8 year sampling period. Average annual productivity (305-419 g C m(-2) year(-1)) and variability around the means (CV = 25-27%) were lower at both the outer nearfield and central nearfield regions of Massachusetts Bay. Annual productivity in 1998 was unusually low at all three sites (< 220 g C m(-2) year(-1)) due to the absence of a winter-spring phytoplankton bloom. Potential factors influencing the occurrence of a spring bloom were investigated. Incident irradiance during the winter-spring period was not significantly different (P <greater than> 0.05) among years (1995-1999). The mean photic depth during the bloom period was significantly deeper (P < 0.05) in 1998, signifying greater light availability with depth. Nutrients were also in abundance during the winter-spring of 1998 with stratified conditions not observed until May. In general, the magnitude of the winter-spring bloom in Massachusetts Bay from 1995 to 1999 was significantly correlated with winter water temperature (r(2) = 0.78) and zooplankton abundance (r(2) = 0.74) over the bloom period (typically February-April). The absence of the 1998 bloom was associated with higher than average water temperature and elevated levels of zooplankton abundance just prior to, and during, the peak winter-spring bloom period.