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 seve ral locations (range: 42 degrees 20.35 ' -42 degrees 26.63 'N; 77 degrees 4 4.19 '- 70 degrees 56.52 'W) in Boston Harbor and Massachusetts Bay, USA, f rom 1995 to 1999, and compared with earlier published data (1992-1994), Pri mary productivity (mg C m(-2) day(-1)) in Massachusetts Bay from 1995 to 19 99 was generally characterized by a well-developed winter-spring bloom of s everal weeks duration, high but variable production during the summer, and a prominent fall bloom. The bulk of production (mg C m(-3) day(-1)) typical ly occurred in the upper 15 m of the water column. At a nearby Boston Harbo r station a gradual pattern of increasing areal production from winter thro ugh summer was more typical, with the bulk of production restricted to the upper 5 m. Annual productivity in Massachusetts Bay and Boston Harbor range d 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 Mas sachusetts Bay. At the harbor station productivity varied more than 3.5-fol d (CV = 40%) over an 8 year sampling period. Average annual productivity (3 05-419 g C m(-2) year(-1)) and variability around the means (CV = 25-27%) w ere lower at both the outer nearfield and central nearfield regions of Mass achusetts Bay. Annual productivity in 1998 was unusually low at all three s ites (< 220 g C m(-2) year(-1)) due to the absence of a winter-spring phyto plankton bloom. Potential factors influencing the occurrence of a spring bl oom were investigated. Incident irradiance during the winter-spring period was not significantly different (P <greater than> 0.05) among years (1995-1 999). The mean photic depth during the bloom period was significantly deepe r (P < 0.05) in 1998, signifying greater light availability with depth. Nut rients were also in abundance during the winter-spring of 1998 with stratif ied conditions not observed until May. In general, the magnitude of the win ter-spring bloom in Massachusetts Bay from 1995 to 1999 was significantly c orrelated with winter water temperature (r(2) = 0.78) and zooplankton abund ance (r(2) = 0.74) over the bloom period (typically February-April). The ab sence of the 1998 bloom was associated with higher than average water tempe rature and elevated levels of zooplankton abundance just prior to, and duri ng, the peak winter-spring bloom period.