A multi-year record of hydrographic and bio-optical properties in the Gulf of Maine: I. Spatial and temporal variability

Abstract

A three year time series of hydrographic and optical data was acquired in the Gulf of Maine using a passenger ferry as a sampling platform; the ferry sails regularly between Yarmouth, Nova Scotia, Canada, and Portland, Maine, USA. The sampling program was designed to provide a time series of spatial information on temperature, salinity, phytoplankton abundance, and inherent optical properties (IOPs) of surface waters. Significant interannual differences were observed in the hydrography. The average vertical temperature gradient over the upper 50 m (measured with XBT’s) was inversely correlated with density at 2 m depth, demonstrating a strong connection between the surface water properties sampled from the ferry and those of the euphotic zone. Highest chlorophyll a concentrations were recorded at hydrographic boundaries, particularly at a frequently-observed offshore extension of the Eastern Maine Coastal Current, south of Penobscot Bay, ME. XBT results showed the intrusion of Maine Bottom Water on time scales of 20–45 d that occurred during late spring or early summer. The optical scattering (at 412 nm) of particulate and dissolved material was highest in the Western Maine Coastal Current (WMCC), but high values were also observed in Scotian Shelf Water (SSW). Typically, spatial patterns of backscattering mirrored total scattering. Acid-labile backscattering (representative of suspended calcium carbonate) was usually 20% of the total particulate backscattering. It reached as high as 80% of the total backscattering during a coccolithophore bloom in 2000, which occurred at the hydrographic boundary between SSW and Jordan Basin Water. Backscattering probability (backscattering/scattering) was usually <1% and showed gradual changes over the season, with lowest values in the late summer/early fall. The shape of the volume scattering function (VSF; defined as the scattered intensity per unit incident irradiance per unit volume) for particulate material was significantly different than that reported by Petzold (1972), and this difference showed coherent variability over time and space. The average chlorophyll a concentration measured over the entire Gulf was positively correlated with the volume of Maine Surface Water present. Dissolved and detrital material was weakly colored and most abundant in the WMCC, and it contained components with significant absorption and scattering. We report a minimal abundance of Case I surface water (i.e., water with inherent optical properties indexed to phytoplankton and their associated constituents) in the Gulf of Maine. The majority of the surface water was Case II for particles, as well as Case II for dissolved and detrital material (where “Case II” indicates water with inherent optical properties dominated by materials other than phytoplankton, such as suspended sediments or colored dissolved organic matter from rivers). Particle-laden Case II water in the Gulf of Maine had a weak but significant correlation with CDOM and detritus-dominated Case II water. The sources of these Case II waters appear to be both inside and outside the Gulf of Maine.