Search

Nitrogen ([N.sub.2]) fixation rates were determined on bulk water collected in the Gulf of Aqaba, Red Sea, during fall (stratified, oligotrophic) and spring (deep mixing, mesotrophic) seasons. [N.sub.2] fixation rates were low yet consistently measurable in both seasons, and maximum rates of 1.0 [+ or -] 0.1 nmol N [L.sup.-1] [d.sup.-1] and 1.9 [+ or -] 0.2 nmol N [L.sup.-1] [d.sup.-1] occurred in the fall and spring, respectively. Amendment with inorganic phosphate did not always increase rates. Rates were uninfluenced by dissolved organic phosphorus treatments. The highest rate (2.1 nmol N [L.sup.-1] [d.sup.-1]) was measured in spring, when surface seawater was amended with an aerosol dust filter. The nifH deoxyribonucleic acid (DNA) gene diversity was analyzed in surface and depth profile samples. Most (58) nifH sequences were similar to the previously identified cluster I (containing sequences from proteobacteria and cyanobacteria) sequences. A small subset of sequences (11) was most similar (>96.5) to nifH nucleotide sequences of cyanobacteria, including Trichodesmium spp. and the unicellular cyanobacterial group A. Sequences similar to cluster III (which contains sequences from many anaerobes) lineages were only retrieved from fall libraries. Quantitative polymerase chain reaction (qPCR) assays were used to estimate the nifH abundance for select phylotypes. Trichodesmium and a [gamma] proteobacteria had maximum abundances at 60-m depth in the water column in fall 2005 (1.4 x [10.sup.5] and 2.3 x [10.sup.2] nifH gene copies [L.sup.-1], respectively). Group A was undetected in all samples, and Trichodesmium and [gamma] proteobacteria were undetected in the spring 2007. The nifH transcription as determined by quantitative reverse-transcription PCR (qRT-PCR) assays was at the detection limit (1-10 transcripts per reaction) for all phylotypes. This study is the first assessment of nifH diversity and rates of 15[N.sub.2] fixation in the Gulf of Aqaba.

In the Gulf of Aqaba, the northeasternmost segment of the Red Sea, phytoplankton blooms are more intense than in other oligotrophic regions (e.g., the Sargasso Sea). In this study, we use multiyear in situ (1988-2000) and Sea viewing Wide Field of View Sensor (SeaWiFS) (1999-2001) chlorophyll a (Chl a) data to describe the dynamics of phytoplankton biomass throughout the Gulf of Aqaba. The temporal pattern of phytoplankton biomass in the Gulf of Aqaba includes a strong spring bloom and a somewhat weaker autumn bloom, the length, intensity, and timing of which vary from year to year. In addition, highly positive west-to-east (W to E) gradients in Chl a were found throughout the Gulf of Aqaba. A corresponding negative gradient in sea surface temperature (SST) obtained from MODerate resolution Imaging Spectroradiometer (MODIS) in 2001 indicates that the gradient of Chl a across the Gulf of Aqaba is an outcome of an Ekman-driven upwelling along the eastern side. Calculations of the upwelling and convective fluxes that are based on meteorological data from Eilat, Israel, indicate that upwelling is comparable to or exceeds convection during much of the year. We present a conceptual model demonstrating how upwelling and convection can either support or oppose each other, thereby jointly controlling mixed-layer depth and the development of phytoplankton blooms. Coastal upwelling plays a larger role in controlling phytoplankton dynamics than was previously thought, and it explains much of the observed spatial and temporal variability in phytoplankton distributions.

Sediment resuspension has a major ecological role in many coastal habitats, lakes, and marine environments. Waves and currents are generally thought to be the major control on sediment resuspension. In this study, the temporal and spatial distributions of suspended sand concentrations (SSAC) were measured at five coral reefs in the northern Gulf of Aqaba, Red Sea, by directly filtering seawater through 100-[micro]m mesh nets. Levels of near-bottom SSAC were twice as high during the day (4.8-9.7 mg [m.sup.-3]) than at night (2.3-5.2 mg [m.sup.-3]). Vertical profiles showed a sharp increase of SSAC (2-6-fold) toward the bottom during the daytime but not at night. The contributions of wind, waves, and currents to variations in SSAC were nonsignificant except during rare southern storms. Temporal and spatial changes in the level of SSAC during the day corresponded with the abundance of benthivorous fish and their activity pattern. Resuspension activities by fish numbered > 1.5 resuspension events [m.sup.-2] [h.sup.-1] and were confined to daytime. An in situ experiment in which fish were excluded from a large section (250 [m.sup.2]) of the reef resulted in elimination of the day-night differences and a significant decrease in daytime SSAC over the treated reef section in comparison to the open, control sites. This study is the first report of a daily pattern of suspended sediment at coral reefs with fish activity as its causative agent.

Effective removal of phytoplankton by the benthic community at a coral reef was found in a study conducted in the Gulf of Aqaba, Red Sea, from 1994-1996. The amount of phytoplankton and chlorophyll was reduced by 15-65% near the reefs compared to adjacent open waters. Pheopigments, the degradation products of chlorophyll, were more concentrated near the reef, where chlorophyll levels were lower. Strong depletion of phytoplankton was found by direct measurements of water passing through a 5-m-long perforated reef.