A comparison of two zooplankton time series data collected in the Gulf of Maine

Interannual and seasonal trends of zooplankton abundance and species composition were compared between the Bongo net and Continuous Plankton Recorder (CPR) time series in the Gulf of Maine. Data from 5799 Bongo and 3118 CPR samples were compared from the years 1978–2006. The two programs use different sampling methods, with the Bongo time series composed of bimonthly vertically integrated samples from locations throughout the region, while the CPR was towed monthly at 10 m depth on a transect that bisects the region. It was found that there was a significant correlation between the interannual (r = 0.67, P < 0.01) and seasonal (r = 0.95, P < 0.01) variability of total zooplankton counts. Abundance rankings of individual taxa were highly correlated and temporal trends of dominant copepods were similar between samplers. Multivariate analysis also showed that both time series equally detected major shifts in community structure through time. However, absolute abundance levels were higher in the Bongo and temporal patterns for many of the less abundant taxa groups were not similar between the two devices. The different mesh sizes of the samplers probably caused some of the discrepancies; but diel migration patterns, damage to soft bodied animals and avoidance of the small CPR aperture by some taxa likely contributed to the catch differences between the two devices. Nonetheless, Bongo data presented here confirm the previously published patterns found in the CPR data set, and both show that the abundance increase of the 1990s has been followed by average to below average levels from 2002 to 06.

Chlorophyll enhancement in the central region of the Bay of Biscay as a result of internal tidal wave interaction

A multi-sensor satellite approach based on ocean colour, sunglint and Synthetic Aperture Radar imagery is used to study the impact of interacting internal tidal (IT) waves on near-surface chlorophyll-a distribution, in the central Bay of Biscay. Satellite imagery was initially used to characterize the internal solitary wave (ISW) field in the study area, where the “local generation mechanism” was found to be associated with two distinct regions of enhanced barotropic tidal forcing. IT beams formed at the French shelf-break, and generated from critical bathymetry in the vicinities of one of these regions, were found to be consistent with “locally generated” ISWs. Representative case studies illustrate the existence of two different axes of IT propagation originating from the French shelf-break, which intersect close to 46°N, − 7°E, where strong IT interaction has been previously identified. Evidence of constructive interference between large IT waves is then presented and shown to be consistent with enhanced levels of chlorophyll-a concentration detected by means of ocean colour satellite sensors. Finally, the results obtained from satellite climatological mean chlorophyll-a concentration from late summer (i.e. September, when ITs and ISWs can meet ideal propagation conditions) suggest that elevated IT activity plays a significant role in phytoplankton vertical distribution, and therefore influences the late summer ecology in the central Bay of Biscay.