Influences of the Gulf of Maine intrusion on the Massachusetts Bay spring bloom: A comparison between 1998 and 2000

The Boston Harbor, Massachusetts Bay and Cape Cod Bay system (MBS) is a semi-enclosed coastal embayment located in the western Gulf of Maine (GOM). The strength of the spring bloom in the MBS varies dramatically and the underlying mechanisms are not well understood. It has been hypothesized that the weak (or missing) 1998 spring bloom was due to increased zooplankton grazing pressure after a relatively warmer winter that led to earlier development of zooplankton populations. However, chlorophyll concentrations were low in the entire GOM region during the spring bloom period of 1998. These low chlorophyll waters would enter the MBS by persistent intrusion from the GOM and affect the MBS spring bloom by reducing the accumulation of phytoplankton biomass, suggesting an alternative explanation for the missing bloom. In this study, the influences of the intruding GOM waters on the MBS spring bloom are examined using numerical simulations and observations to compare two contrasting spring blooms (a weak bloom in 1998 versus a strong bloom in 2000). The results indicate that intruding waters from the GOM significantly reduced the strength of spring bloom in 1998, but enhanced the spring bloom in 2000. A theoretical analysis suggests that the influence of the intrusion on the MBS spring bloom is mostly limited to the northern portion of the MBS, and the impact area is determined by local net growth, velocity of intruding flow and horizontal mixing. The intrusion of GOM waters carries the signal of long-term variability in the GOM region and thus may be an important oceanic pathway for climate changes to impact the MBS ecosystem.