Distribution and diel vertical movements of mesopelagic scattering layers in the Red Sea

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Marine Biology 159 (2012): 1833-1841, doi:10.1007/s00227-012-1973-y.The mesopelagic zone of the Red Sea represents an extreme environment due to low food concentrations, high temperatures and low oxygen waters. Nevertheless, a 38 kHz echosounder identified at least four distinct scattering layers during the daytime, of which the 2 deepest layers resided entirely within the mesopelagic zone. Two of the acoustic layers were found above a mesopelagic oxygen minimum zone (OMZ), one layer overlapped with the OMZ, and one layer was found below the OMZ. Almost all organisms in the deep layers migrated to the near-surface waters during the night. Backscatter from a 300 kHz lowered Acoustic Doppler Current Profiler indicated a layer of zooplankton within the OMZ. They carried out DVM, yet a portion remained at mesopelagic depths during the night. Our acoustic measurements showed that the bulk of the acoustic backscatter was restricted to waters shallower than 800 m, suggesting that most of the biomass in the Red Sea resides above this depth.This research is based in part on work supported by Award Nos. USA 00002, KSA 00011 and KSA 00011/02 made by KAUST to the Woods Hole Oceanographic Institution

Phytoplankton Dynamics within a Discrete Water Mass off Cape Hatteras, North Carolina: the Lagrangian Experiment

As part of the Department of Energy Ocean Margins Program, we examined the spatial and temporal variability in primary production and phytoplankton pigments during two cruises to the shelf waters between the Chesapeake Bay and Cape Hatteras. The first cruise was conducted in March 1996, reflecting well-mixed conditions just prior to the spring transition, while the second cruise was conducted during July 1996 when the water column was more stratified. During each cruise, primary production and high performance liquid chromatography (HPLC) pigments for the whole community and \u3c8-mum size fraction were determined for several successive days within a discrete water mass by following a near-surface tracking drogue. In March, production ranged from 0.50 to 0.65 g C m(-2) d(-1), with 52-62% of the production attributed to the smaller size fraction. About 50% of the total chlorophyll a (chl a) was found in the \u3e 8-mum size fraction. Pigment composition was dominated by chlorophylls a, c(1) and c(2), and fucoxanthin, indicating the importance of diatoms. In July, production was more variable, ranging from 0.38 to 2.09 g C m(-2) d(-1), with 41-83% of production attributed to the \u3c8-mum size fraction. Rates increased over the 4-day study. Most of the chi a was found in the \u3c8-mum size fraction. The phytoplankton pigments were dominated by chi a and fucoxanthin with chlorophylls c(1) and c(2), again indicating the importance of diatoms during this cruise.CHEMTAX (Mackey et al. CHEMTAX User\u27s Manual: a program for estimating class abundances from chemical markers-application to HPLC measurements of phytoplankton pigments. CSIRO Marine Laboratories, Report 229, Hobart, 42 pp.), a factor analysis computer program, was used to examine phytoplankton community structure using marker pigments from our HPLC analyses to determine the relative importance of various taxa. In March, diatoms dominated whole water samples, with consistent contributions from dinoflagellates and cryptophytes. The \u3c8-mum fraction was dominated by small diatoms, chrysophytes, cryptophytes and dinoflagellates. In July, diatoms were still present and important, but prymnesiophytes, cryptophytes and cyanobacteria contributed in both size classes. Correlation analyses indicated that primary production was positively correlated with light and temperature. Chl a biomass was positively correlated with the concentrations of NO2 + NO3 and negatively correlated with temperature. These correlations support the observation that temperature played a major role in the phytoplankton dynamics in this shelf ecosystem. (C) 2002 Elsevier Science Ltd. All rights reserved