Biological validation of acoustic backscatter and observations of biological/hydrographic interactions at the mesoscale

The objective of this thesis was to study the distributions of zooplankton at the mesoscale, using acoustic backscatter data (from an Acoustic Doppler Current Profiler, ADCP and SIMRAD EK500 echosounder) taken concurrently with hydrographic data (from SeaSoar and CTDs) and net-sampled zooplankton (from a Longhurst Hardy Plankton Recorder, LHPR). A further aim was to validate acoustic backscatter data with concurrent net data. Two cruises collected multidisciplinary datasets from two contrasting regions. RRS Discovery cruise 209 to the Arabian Sea (Indian Ocean) in August 1994 and RRS Discovery cruise 224 to the Alboran Sea (Mediterranean Sea) during November and December 1996. The Arabian Sea was dominated by a strong oxygen minimum zone (OMZ) that extended from the thermocline (~50 m), to ~1100 m. From the analysis of sound scattering layers and net samples the OMZ appeared to play a major role in determining the vertical distribution of zooplankton. A persistent layer of high acoustic backscatter and high zooplankton biovolume indicated that the vertical extent of some zooplankton was restricted to oxygenated surface waters. Euphausiids, decapods and myctophid fish were observed to undertake dawn diel migration between the surface and depths of 300 - 400 m, well into suboxic waters, whilst the vertical distribution of polychaetes appeared to be independent of oxygen concentration and it is suggested that these taxa may possess morphological and physiological adaptations to low oxygen environments. The Alboran Sea is the site of an intense geostrophic density from that has associated ageostrophic vertical components. It appeared that the front was exhibiting a fertilising effect on both phytoplankton and zooplankton. Observations of a layer of chlorophyll a fluorescence coincident with subducted surface waters indicated that phytoplankton were down and along isopycnals to a depth of ~200 m. Acoustic backscatter data and net samples indicated that increased numbers of euphausiids and chaetognaths occurred coincident with the drawn-down phytoplankton. Smaller zooplankton, copepods and possibly euphausiid larvae, which did not undertake diel migration, remained concentrated near the surface in the fast-flowing frontal jet.