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Surface aggregations of the diazotrophic cyanobacterium Trichodesmium were encountered at nearly every station along a 300-km transect in the northwestern Gulf of Mexico during July 2000. 15 N 2 -fixation rate measurements for all treatments and locations ranged from 1.3 X 10 -4 nmol N trichome -1 h -1 to 2.6 X 10 -2 nmol N trichome -1 h -1 , with a mean of 3.4 X 10 -3 ± 7.6 X 10 -4 nmol N trichome -1 h -1 (mean ± SE, n = 45). Trichodesmium C-fixation, measured by the incorporation of 13 C, ranged from 1.3 X 10 -2 nmol C trichome -1 h -1 to 4.5 X 10 -1 nmol C trichome -1 h -1 , with a mean of 1.2 X 10 -1 ± 9.3 X 10 -2 nmol C trichome -1 h -1 (mean ± SE, n = 44). Both 15 N 2 -fixation and 13 C-fixation rates attenuated with decreasing light intensity from a maximum at 50% surface intensity. Areal Trichodesmium 15 N 2 -fixation rates, based on a 10-h N 2 -fixation day, enumerated vertical trichome abundance and corrected for estimated in situ light intensity ranged from 47.2 μmol N m -2 d -1 to 118.6 μmol N m -2 d -1 , with a mean of 84.5 ± 17.7 μmol N m -2 d -1 (mean ± SE, n = 4). The light natural abundance isotopic signatures of dissolved inorganic nitrogen, particulate nitrogen, and zooplankton nitrogen within these surface populations are consistent with our limited biological rate measurements and indicate that recently fixed N is moving into the food chain to promote secondary production in these nutrient-impoverished waters. Although our sample size was relatively small, natural abundance isotope data indicate that as much as 60% of macrozooplankton C and N was derived from Trichodesmium in the oligotrophic regions of the Gulf of Mexico.

Traditional field techniques to monitor water quality in large estuaries, such as boat-based surveys and autonomous moored sensors, generally provide limited spatial coverage. Satellite imagery potentially can be used to address both of these limitations. Here, we show that satellite-based observations are useful for inferring total-suspended-solids (TSS) concentrations in estuarine areas. A spectra-matching optimization algorithm was used to estimate the particle backscattering coefficient at 400 nm, b bp (400), in Chesapeake Bay from Sea-viewing Wide-Field-of-view Sensor (SeaWiFS) satellite imagery. These estimated values of b bp (400) were compared to in situ measurements of TSS for the study period of September 1997–December 2003. Contemporaneous SeaWiFS b bp (400) values and TSS concentrations were positively correlated ( N = 340, r 2 = 0.4, P b bp (400) values served as a reasonable first-order approximation for synoptically mapping TSS. Overall, large-scale patterns of SeaWiFS b bp (400) appeared to be consistent with expectations based on field observations and historical reports of TSS. Monthly averages indicated that SeaWiFS b bp (400) was typically largest in winter (>0.049 m −1 , November–February) and smallest in summer ( −1 , June–August), regardless of the amount of riverine discharge to the bay. The study period also included Hurricanes Floyd and Isabel, which caused large-scale turbidity events and changes in the water quality of the bay. These results demonstrate that this technique can provide frequent synoptic assessments of suspended solids concentrations in Chesapeake Bay and other coastal regions.

Using Tampa Bay, FL as an example, we explored the potential for using MODIS medium-resolution bands (250- and 500-m data at 469-, 555-, and 645-nm) for estuarine monitoring. Field surveys during 21–22 October 2003 showed that Tampa Bay has Case-II waters, in that for the salinity range of 24–32 psu, (a) chlorophyll concentration (11 to 23 mg m−3), (b) colored dissolved organic matter (CDOM) absorption coefficient at 400 nm (0.9 to 2.5 m−1), and (c) total suspended sediment concentration (TSS: 2 to 11 mg L−1) often do not co-vary. CDOM is the only constituent that showed a linear, inverse relationship with surface salinity, although the slope of the relationship changed with location within the bay. The MODIS medium-resolution bands, although designed for land use, are 4–5 times more sensitive than Landsat-7/ETM+ data and are comparable to or higher than those of CZCS. Several approaches were used to derive synoptic maps of water constituents from concurrent MODIS medium-resolution data. We found that application of various atmospheric-correction algorithms yielded no significant differences, due primarily to uncertainties in the sensor radiometric calibration and other sensor artifacts. However, where each scene could be groundtruthed, simple regressions between in situ observations of constituents and at-sensor radiances provided reasonable synoptic maps. We address the need for improvements of sensor calibration/characterization, atmospheric correction, and bio-optical algorithms to make operational and quantitative use of these medium-resolution bands.

The NASA Experimental Advanced Airborne Research Lidar (EAARL), a temporal waveform-resolving, airborne, green wavelength LIDAR (light detection and ranging), is designed to measure the submeter-scale topography of shallow reef substrates. Topographic variability is a prime component of habitat complexity, an ecological factor that both expresses and controls the abundance and distribution of many reef organisms. Following the acquisition of EAARL coverage over both mid-platform patch reefs and shelf-margin bank reefs within Biscayne National Park in August 2002, EAARL-based optical indices of topographic variability were evaluated at 15 patch reef and bank reef sites. Several sites were selected to match reefs previously evaluated in situ along underwater video and belt transects. The analysis used large populations of submarine topographic transects derived from the examination of closely spaced laser spot reflections along LIDAR raster scans. At all 15 sites, each LIDAR transect was evaluated separately to determine optical rugosity (Rotran), and the average elevation difference between adjacent points (Av(δEap)). Further, the whole-site mean and maximum values of Rotran and Av(δEap) for the entire population of transects at each analysis site, along with their standard deviations, were calculated. This study revealed that the greater habitat complexity of inshore patch reefs versus outer bank reefs results in relative differences in topographic complexity that can be discerned in the laser returns. Accordingly, LIDAR sensing of optical rugosity is proposed as a complementary new technique for the rapid assessment of shallow coral reefs.

In May 1992, surface seawater samples were collected along an equatorial transit (130 to 100°W) and analyzed for total hydrogen ion concentration (expressed as spectrophotometric pHT) total dissolved inorganic carbon (coulometric CT), and total alkalinity (potentiometric AT and spectrophotometric AT). This data set, which presents a striking view of the chemical signature of tropical instability waves, is also unusual in that its “overdetermination” of the CO2-system in seawater includes both potentiometric AT data and the first spectrophotometric AT data collected at sea using a double-wavelength, one-step acid addition method. Our data set indicates that spectrophotometric AT measurements (ATspec) are both precise (±1.8 μmol/kg) and accurate; the mean observed difference between ATspec and AT obtained potentiometrically in this study (ATpot) is 1 μmol/kg. As only two of the three measured parameters are required to characterize the CO2-system in seawater, this analytical redundancy of our analyses (CT, AT, pH) afforded an opportunity to evaluate, in terms of thermodynamic consistency, a recent calibration of m-cresol purple, a pH indicator. Using various carbonic acid dissociation constants, measured parameters were combined in pairs (CT-AT, pH-CT, and pH-AT) to calculate a third parameter for comparison with the shipboard measurements. Depending on the selected set of carbonic acid dissociation constants, the average offset between directly measured and predicted values of AT and CT was as small as ±1 μmol/kg. The results of this study indicate that the present 25°C mCP calibration, paired with the 25°C combined dissociation constants of Hansson and Mehrbach, produces accurate predictions of AT and CT. Extensive replication of the shipboard spectrophotometric measurements made it possible to examine directly the sensitivity of derived parameters to variations or errors in input AT and pH. In accord with earlier theoretical treatments of this question, the results presented demonstrate that pH is imprecisely predicted from AT and CT, whereas imprecision in measured pH—at the level typical of spectrophotometric measurements (±0.0004)—contributes negligibly (±0.3 μmol/kg) to imprecision in derived AT and CT. In view of the high precision of pHTspec measurements and both the precision and reliability of CT measurements (supported by the use of certified SIO reference materials) pHTspec and CTcoul constitute a particularly useful pair of parameters for shipboard study of the oceanic carbon dioxide system. The results indicate, as well, that the AT-pHTspec pairing is of particular interest in ocean regions where simple AT vs salinity relationships are observed. In light of recent advances, the role of pH measurements in CO2-system characterizations should be re-evaluated. Spectrophotometric measurements of pH have much to contribute in documenting the oceans' evolving response to anthropogenic C02.

Tropical instability waves have been shown to have a major impact on the variability of temperature and nutrients along the equatorial wave guide. In order to assess the impact of these features on carbon species distributions during an ENSO event, sea surface temperature, salinity, sigma-t, nitrate, CO[sub 2] fugacity, total inorganic carbon, total alkalinity, and pH along the equator were measured from 130[degrees]W to 100[degrees]W during 8-15 May 1992. Concurrent moored measurements of surface currents and temperature were also made at 0[degrees], 110[degrees]W. Results indicate that tropical instability waves, with periods of 15 to 20 days and zonal wavelengths of 700-800 km, controlled the observed spatial variability of the CO[sub 2] species, nitrate and hydrographic parameters at the equator. 37 refs., 3 figs.

During a recent NOAA JGOFS Equatorial Pacific cruise all four analytical parameters of the carbonate system were measured—pH, total alkalinity (TA), total carbon dioxide (TCO2), and the fugacity of carbon dioxide (fCO2). The measurements made during leg 2 on surface waters have been used to examine the internal consistency of the carbon dioxide system in these waters. The internal consistency of the measurements was examined by using various inputs of the measured parameters (pHTA, pHTCO2, pHfCO2, fCO2TA, fCO2TCO2 and TATCO2) to calculate the components of the CO2 system. The results indicate that the measurements have an internal consistency of ±0.003–0.006 in pH, ±5–7 μmol kg−1 in TA, ±5–7 μmol kg−1 in TCO2 and ±6–9 μAtm in fCO2 if reliable constants are used for the dissociation of carbonic acid in seawater. These results indicate that our present understanding of the thermodynamics of the carbonate system in seawater is close to the present accuracy in measuring the various parameters of the system (±0.002 in pH, ±4 μmol kg−1 in TA, ±2 μmol kg−1 in TCO2 and ±2 μAtm in fCO2).

Spectrophotometric open-ocean seawater pH measurements are simple, fast and precise. The sulfonephthalein indicator m-cresol purple (mCP) is recommended for open-ocean surface-to-deep pH measurements. The salinity- and temperature-dependence of such measurements (293⩽T⩽303 and 30⩽S⩽37) is given as: pHT(sw) = 1245.69T + (2.11 × 10−3)(35 − S) + log(R − 0.00692.2220 − R0.1330) on the total hydrogen ion concentration scale ([H+]T = [H+]f + [HSO4−]), in units of mol·(kg-soln)−1. R is the ratio of indicator absorbances at molar absorptivity maxima (i.e. R + 578A/434A). The at-sea analytical precision of the technique, evaluated on two recent NOAA cruises, is approximately 0.0004 pH units.

High resolution seismic reflection profiling within Discovery Bay, Jamaica has revealed that this 54 m deep, subcircular, reef-protected coastal basin has over 100 m of relief. Partially buried reefal structures at a water depth of −40 m indicate a past sea-level stillstand. In addition, side scan sonar reveals that the slopes leading down into the basin from these upper reefs are nearly acoustically featureless indicating little downslope movement by reefal debris and no slope failure. The basin center is dominated by a 15 m thick upper sequence of mollusk-rich carbonate mud which has accumulated at a rate of 1600 mm 1000 yrs for the past 9000 yrs based upon 14 C dates. The total 50 m of basinal infilling consists of two primary sequences both approximately of the same thickness. Without very long cores (> 15 m) we can only speculate on basin infilling history. The deepest subsurface portion of the basin lies on line with a fault-controlled, dry valley and a pronounced change in the seaward reef trend. The north coast of Jamaica consists of a terraced, limestone coastal plain. We propose that Discovery Bay initially formed by dissolution from calcite-undersaturated groundwaters focused along this fault. A reefal barrier rim developed and sealed off this depression from the open Caribbean Sea sometime during the Pleistocene. A similar coastal depression, Rio Bueno, lies just to the west of Discovery Bay, but remained open probably due to continued freshwater influx which retarded reef growth during lowstands throughout the late Neogene and Quaternary.