Temporal and vertical variability in optical properties of New England shelf waters during late summer and spring

Abstract

Abstract. Relationships between optical and physical properties were examined on the basis of intensive sampling at a site on the New England continental shelf during late summer 1996 and spring 1997. During both seasons, particles were found to be the primary source of temporal and vertical variability in optical properties since light absorption by dissolved material, though significant in magnitude, was relatively constant. Within the particle pool, changes in phytoplankton were responsible for much of the observed optical variability. Physical processes associated with characteristic seasonal patterns in stratification and mixing contributed to optical variability mostly through effects on phytoplankton. An exception to this generalization occurred during summer as the passage of a hurricane led to a breakdown in stratification and substantial resuspension of nonphytoplankton particulate material. Prior to the hurricane, conditions in summer were highly stratified with subsurface maxima in absorption and scattering coefficients. In spring, stratification was much weaker but increased over the sampling period, and a modest phytoplankton bloom caused surface layer maxima in absorption and scattering coefficients. These seasonal differences in the vertical distribution of inherent optical properties were evident in surface reflectance spectra, which were elevated and shifted toward blue wavelengths in the summer. Some seasonal differences in optical properties, including reflectance spectra, suggest that a significant shift toward a smaller particle size distribution occurred in summer. Shorter timescale optical variability was consistent with a variety of influences including episodic events such as the hurricane, physical processes associated with shelfbreak frontal dynamics, biological processes such as phytoplankton growth, and horizontal patchiness combined with water mass advection.

Contributions of phytoplankton and other particles to inherent optical properties in New England continental shelf waters

“…levels existed throughout the water column and light level may have had a larger effect on cell size. The presence of larger cells at high light levels in spring surface waters is supported by previous studies using phytoplankton cultures, which have suggested that nutrient-replete cells are generally larger at high light levels than at low light levels (Falkowski et al 1985;Sakshaug et al 1987;Sosik et al 1989). In addition, species differences could have contributed to changes in D with depth.…”

“…Eukaryotic phytoplankton and Synechococcus decreased in abundance with depth in both seasons ( Table 2). The amplitude of the change was greater in the summer, consistent with a larger change in nutrient availability with depth than in the spring (Sosik et al 2001). Cells increased in size with depth in the summer, in contrast to the spring when cells decreased in size with depth (Table 2).…”

“…The water column was well stratified in the summer (before the passage of Hurricane Edouard), with depleted nutrient levels in the surface mixed layer, a subsurface chlorophyll maximum between 20 and 30 m, and midwater column maxima in a p and b p (Sosik et al 2001). In the spring, the water column was less stratified; stratification increased over the 3-week sampling period, leading to a phytoplankton increase in surface waters, an associated decrease in nutrient levels, and increased a p and b p (Sosik et al 2001). In the spring, the highest values of chlorophyll a, a p , and b p consistently occurred in the top 25 m of the water column.…”

“…During this experiment, we intensively sampled individual particle and bulk optical properties at a site on the New England continental shelf during summer and spring. We showed in previous work that particles (as opposed to CDOM) were the primary source of temporal and vertical variability in optical properties (Sosik et al 2001); in the present study we determine the optical contributions of distinct particle groups. We combine Mie theory and flow cytometric (FCM) measurements to determine the particle properties and contributions to IOPs at 488 nm of eukaryotic pico/nanophytoplankton (''eukaryotic phytoplankton''), Synechococcus, heterotrophic prokaryotes, detritus, and minerals.…”

“…Bulk optical properties-Vertical profiles for water sampling and measurements of optical properties were made as part of CMO as described by Sosik et al (2001). The CMO site was located on the southern New England shelf, south of Martha's Vineyard (40Њ30ЈN, 70Њ30ЈW), in a region known as the ''Mud Patch'' and with a water depth of ϳ70 m. Data were collected during two 3-week cruises in the late summer of 1996 aboard the R/V Seward Johnson (cruise SJ9610, 17 August-7 September) and spring of 1997 aboard the R/V Knorr (cruise KN150, 24 April-13 May).…”