Unraveling phytoplankton optical variability in the Gulf of Maine during the spring and fall transition period

The hydrographic and optical properties in the Gulf of Maine (GOM) are controlled by an array of forces operating on local scales such as the distributed river system to global such as the North Atlantic Oscillation. Regional hydrographic and bulk optical relationships, however, are not always in concert revealing that variable forcing mechanisms operate separately on different water column characteristics leading to a complex set of interrelationships that vary on both vertical and horizontal spatial scales. In this study, we examined hydrographic and bio-optical variability during two seasonal surveys with an emphasis on the influence of regional phytoplankton variability on ocean color. mponent optical variability suggested that the relatively invariant green ocean color of the GOM is related to the temporal and spatial invariance of the total absorption signal. This invariance, however, masked both variations in the relative magnitudes of the dissolved and particulate optical components (e.g. CDOM, phytoplankton, and non-algal particles) and the optical variations observed within these components. Component absorption variability between provinces of the GOM and between seasonal surveys was distinct with discernible differences. These temporal and regional differences were due in part to particle type and phytoplankton absorption spectral variability. Variability in phytoplankton size-class dominance in both fall and spring implied that phytoplankton size dependencies develop in response to variability in mixing, stratification, and stability which may occur independently within different regions of the GOM. However, depth dependent photoacclimation was not a driving factor indicating that mixing rates exceeded photoacclimation rates during both cruises.