The purpose of the cruise was to make time-series profiles of optics, particles, colored dissolved organic matter, biological components and turbulence near the central mooring site at 40 30'N 70 30'W.
Four or five days prior to our cruise there was an intense nor'easter that passed over the CMO mooring site, followed by calm, sunny weather. As we headed to the central CM&O site on April 23, a strong gale was building. We managed to make one CTD profile and deploy the Agrawal/Hill tripod and a sediment trap mooring before the seas built up too much. After another CTD profile the next morning conditions did not allow us to work. At that time there was a strong subsurface chlorophyll peak and a well-mixed bottom layer. By the time we resumed CTD sampling 48 hrs later, a slightly different surface water mass had moved in with temperatures dropping from 7 to 6 deg. C and salinity dropping by 0.1 psu.
The water column was weakly stratified, particularly near the bottom, where there was often a 20 m thick bottom boundary layer. This boundary layer behaved much like a surface mixed layer, except the turbulence was driven by the tides. When tides were slack, the bottom layer became weakly stratified. As the tide increased, turbulence grew upward from the bottom, much as nightly convection deepens the surface layer into the weakly stratified water above the seasonal thermocline. Temperature and salinity in the bottom boundary layer remained surprisingly constant throughout the 19 days on station with a temperature of 5.9-6.0 deg C and a salinity of 32.30-32.37 psu, suggesting that a large region of bottom water was well-mixed and that the tidal motions never moved this water out of the site. Light attenuation, absorption and scattering measurements showed the bottom boundary layer was always well-mixed in particulate matter. The optical backscattering sensors on the turbulence profilers gave profiles similar to some of the optical attenuation data taken during the day. Particle concentrations increased as the bottom boundary layer became more turbulent, but at this point we see no obvious relation to the intensity of surface storms, suggesting tidal mixing is important. Any material that was resuspended seemed to be quickly dispersed throughout the bottom mixed layer and then stopped beneath the density step capping the bottom layer, but the variations in bottom particle concentrations was not large. Sediment cores had a mat-like surface not seen last September and may have inhibited resuspension as suggested by previous studies.
A similar cycle of mixing appeared in the surface layer, which was as much as 30 m thick as several late winter storms passed over the area, but we had difficulty determining whether the mixing was produced by convection or by the tides. Most of the mixing was driven by surface fluxes - wind stress and convection, but some turbulence in stratified water just below the surface may have been produced by tidal shear. Surface temperatures gradually warmed from 6 deg C on day 3 to 8.5 deg C on day 19, while salinity gradually freshened slightly from 32.3 to 32.05 psu. A subsurface chlorophyll maximum existed on the first day and then waned and waxed as storms increased the depth of mixing and calm sunny days enhanced primary production.
Chlorophyll concentration was generally higher in the upper part of the water column (up to 2.75 mg/l) than in deeper waters where concentrations were as low as 0.25 mg/l. Phaeopigment concentrations were on average about a third of the chlorophyll concentrations in the surface layer and from half to equal to the chlorophyll concentrations in the lower part of the water column. Particle concentrations were found to be at the limit of detectability by both the LISST in situ measurements and the Galai. On the Galai screen, a lot of protozoans and ciliates were observed but particle counts were generally small (maximum of 4,000/ml with average modal diameter about 2 micrometers). With both instruments, particle counts were higher in the surface layer than at depth. Similar results were obtained with the Coulter Multisizer.
The surface and bottom boundary layers never merged, and the density cap on the BBL intensified with time. The midwater developed a vertical gradient in temperature with time, but salinity remained well mixed, suggesting an intrusion of water from another source rather than local mixing. The inherent optical properties showed that particle concentrations remained low in the mid water, nearly always exhibiting a distinct minimum.
The low stratification observed during the spring CMO cruise was unable to support the large solitons observed during the fall cruise. The lack of internal waves greatly reduced the short-term (< 1 hr) variability in the optical properties of the water column during most of the cruise. However, the time series ended at about spring tide and we began to see evidence of internal waves. Signatures on the BioSonics showed groups of low-frequency waves displacing the density step capping the bottom boundary layer. Water above the interface appeared full of diffuse scatterers. Occasionally consecutive casts showed large excursions in the BBL thickness.
We detected substantial differences in the optical properties of the water column and particulate material near the central CM&O mooring site between late summer 1996 and spring 1997. Diffuse attenuation coefficients were about 2 times higher in the mixed layer in the spring and decreased consistently with depth. In contrast, during the summer sampling period, there were strong subsurface maxima diffuse attenuation coefficients, associated with high phytoplankton pigment concentrations in the middle of the water column and high particle loads in the bottom boundary layer. In addition, temporal variations in the vertical structure of attenuation were much smaller in the spring.
The phytoplankton community was generally shifted towards larger cell sizes during spring. In surface waters, the abundance of picoplankton was approximately 5 times lower than in the summer, while concentrations of larger eukaroytic phytoplankton (2-50 micrometer diameter) were about twice as high. The composition of the largest phytoplankton size classes varied over the 3 week cruise. Several species of the dinoflagellate Ceratium were present throughout, with initially high abundances of the diatom Coscinodiscus. Concentrations of these diatoms decreased dramatically after the first storm, while chains of the diatom Chaetoceros sp. became abundant.
The integrated biomass of phytoplankton (calculated from measurements of individual cell light scattering) was higher by about five-fold in the spring, consistent with the higher surface pigment concentrations observed during this cruise. Preliminary analysis indicates that primary production was also higher in the spring.
At the end of the cruise we made a transect of stations from 39 50' N (just south of the shelf break) to 40 50' N (50 m isobath), choosing station locations based on the sections made a few days earlier by Jack Barth et al. with SeaSoar on the Endeavor and found similar strong gradients vertically and laterally in all parameters.
Scientific Party