Kamazima M. M. Lwiza

Phosphorus limitation of nitrogen fixation by Trichodesmium in the central Atlantic Ocean

Marine fixation of atmospheric nitrogen is believed to be an important source of biologically useful nitrogen to ocean surface waters, stimulating productivity of phytoplankton and so influencing the global carbon cycle. The majority of nitrogen fixation in tropical waters is carried out by the marine cyanobacterium Trichodesmium, which supplies more than half of the new nitrogen used for primary production. Although the factors controlling marine nitrogen fixation remain poorly understood, it has been thought that nitrogen fixation is limited by iron availability in the ocean. This was inferred from the high iron requirement estimated for growth of nitrogen fixing organisms and the higher apparent densities of Trichodesmium where aeolian iron inputs are plentiful. Here we report that nitrogen fixation rates in the central Atlantic appear to be independent of both dissolved iron levels in sea water and iron content in Trichodesmium colonies. Nitrogen fixation was, instead, highly correlated to the phosphorus content of Trichodesmium and was enhanced at higher irradiance. Furthermore, our calculations suggest that the structural iron requirement for the growth of nitrogen-fixing organisms is much lower than previously calculated. Although iron deficiency could still potentially limit growth of nitrogen-fixing organisms in regions of low iron availability—for example, in the subtropical North Pacific Ocean—our observations suggest that marine nitrogen fixation is not solely regulated by iron supply.

The effects of channels and shoals on exchange between the Chesapeake Bay and the adjacent ocean

The role of bathymetric changes in determining the transport of water and salt in the lower Chesapeake Bay (LCB) was investigated using high-resolution acoustic Doppler current profiler (ADCP) and conductivity-temperature-depth profiles. A cross-channel transect was repeated eight times during neap tides on October 6–7, 1993, which illustrated the lateral structure of the longitudinal and transverse flow fields and the intratidal variations in the flow structure across the LCB. Amplitude and phase of the M2 tidal component, as well as the mean flow velocity, were calculated using least squares fitting at every point of a uniform grid obtained from the ADCP data. The results differ from the classical two-layer pattern of estuarine circulation modified by Coriolis effects but are consistent with recent hydrographic observations. Semidiurnal flow was highest over the navigational channels, and lateral gradients were strongest in regions of sharp bathymetric changes. The phase lag of the semidiurnal flow also showed lateral and vertical gradients that represented advances at the bottom with respect to the surface and over the shoals in relation to the channels. The section of the water column measured indicated a mean outflow of 0.7×104 m3/s and a mean inflow of 1.3×104 m3/s. The apparent gain of water by the estuary during the period of observation can be explained by meteorologically forced net barotropic inflow. The depth-averaged mean longitudinal flow consisted of inflow in the navigational channels and outflow over the shoals. The mean transverse flow showed near-surface convergence over the channels. We propose a possible explanation for the observed flow and density structure as follows: the barotropic and baroclinic forcing interact with the bathymetry to extend the inflow from the bottom to the surface, thereby inducing a transverse circulation that yields near-surface convergence over the channels.

Fortnightly variability in the transverse dynamics of a coastal plain estuary

Current velocity and water density profiles were obtained along two cross-estuary transects with the purpose of determining the fortnightly variability of the transverse dynamics in a partially stratified coastal plain estuary. The profiles were measured with a towed acoustic Doppler current profiler and a conductivity-temperature-depth recorder in the James River estuary, Virginia. The cross-estuary transects were sampled during the spring tides of October 26–27, 1996, and the ensuing neap tides of November 2–3, 1996. The transects were ∼4 km long, featured a bathymetry that consisted of a channel flanked by shoals, and were sampled repeatedly during two semidiurnal tidal cycles (25 hours) in order to separate semidiurnal, diurnal, and subtidal signals from the observations. This work concentrates on the subtidal transverse dynamics. The transverse baroclinic pressure gradients were larger during neap tides than during spring tides. During spring tides the advective accelerations were predominantly greater than the Coriolis accelerations, most markedly over the edges of the channel. Both effects combined with frictional influences to balance the pressure gradient in the transverse direction. During neap tides, advective accelerations were not as dominant over Coriolis accelerations as during spring tides. Also, during neap tides, Coriolis played a more relevant role, compared to spring tides, in combining with friction to balance the pressure gradient. This behavior was indicative of the momentum balance approaching gravitational circulation modified by the Earths rotation, weak friction, and nonlinear advection during neap tides. The balance became more influenced by nonlinear advection and friction and less influenced by the Earths rotation during spring tides. These results showed that transverse dynamics of a partially stratified estuary are far from being in geostrophic balance.

Convergence of lateral flow along a coastal plain estuary

A set of velocity profiles obtained in the James River estuary with an acoustic Doppler current profiler was used in combination with the results of an analytic tidal model to depict the appearance of surface lateral flow convergences (∂v/∂y) during both flood and ebb stages of the tidal cycle. The bathymetry of the estuary was characterized by a main channel and a secondary channel separated by relatively narrow shoals. Lateral surface flow convergences appeared over the edges of the channels and were produced by the phase lag of the flow in the channel relative to the shoals. Flood convergences developed in the late tidal stages and ebb convergences appeared soon after maximum currents. Most of these convergences caused fronts in the density field and flotsam lines that also appeared over the edges of the channel and that lasted <2 hours. The transverse flows associated with the convergences were mostly in the same direction throughout the water column. In fact, the vertically averaged flow produced the same convergence patterns as those near the surface. The analytic tidal model reproduced well the timing and location of the convergences as observed in the James River. Model results with different bathymetry emulated the results in other estuaries, e.g., axial convergence in an estuary with a channel in the middle. This work showed that the strength of lateral convergences along the estuary was proportional to the tidal amplitude and the channel steepness. It also suggested that the convergences were produced mainly by the tidal flow interacting with the channel-shoal bathymetry, i.e., that they did not require the presence of density gradients. However, the analytic model underestimated the magnitude of the convergences and did not account for vertical circulations associated with fronts. The formation of fronts resulted from the interaction of the tidal flow with the bathymetry and the density field.

Separating baroclinic flow from tidally induced flow in estuaries

A simple method is used to separate the tidally induced and density-driven subtidal flows in a coastal plain estuary. This method is applicable to weak wind conditions and to systems with appreciable fortnightly variation of tidal amplitude. The baroclinic density-driven motion is assumed to depend on the river discharge, which generates a horizontal density gradient, and is weakened by vertical mixing, which in turn depends on tidal forcing. The barotropic tidally induced motion is assumed to be a function of the tidal amplitude. By Taylor series expansions, two equations are obtained. These equations show the dependence of the tidally induced flow component on the tidal amplitude and the dependence of the density-driven flow component on the ratio between river discharge and tidal amplitude, respectively. The method is applied to water velocity data obtained in the James River, Virginia, in October-November 1996. The data cover two spring tidal cycles and two neap tidal cycles. The vertical structures, as well as the depth mean, of both tidally induced and density-driven components of the subtidal flow are obtained. Results show that the tidally induced component has a predominant seaward flow in the channel and a landward flow over the shoals. The density-driven exchange flow is seaward over the shoals and landward in the channel. These results are consistent with theoretical model results which show that the tidally induced component and density-driven component compete against each other. The increased tidal mixing and tidally induced exchange flow during spring tides reduce density-driven motion, which results in a weak net subtidal flow. In contrast, during neap tides, both the tidally induced flow component of the subtidal flow and tidal mixing are weak, and the tidally induced flow is overwhelmed by the density-driven flow component, which results in a stronger subtidal flow. By extending the proposed method, we suggest that future studies use a least squares fitting technique to obtain an optimal estimate for the tidally induced and density-driven subtidal flow components.

Multivariate objective analysis of the coastal circulation of Barbados, West Indies: implication for larval transport

A multivariate spatial objective analysis (MVOA) assimilating high spatio-temporal resolution of hydrographic (CTD) and acoustic (ADCP) observations near Barbados provided a comprehensive view of the local surface circulation (0–100 m) during early spring of two consecutive years (1996 and 1997). Significant submesoscale fluctuations of the velocity and salinity fields exhibit a very dynamic environment. In the middle of each cruise, lowsalinity water originating from the Amazon and entrained by a North Brazil Current Ring (NBCR) intruded from offshore and persisted on the west coast of Barbados throughout the rest of the survey. Principal component analysis (PCA) of velocity relative to the vertical structure and temporal factors in the study area demonstrated that the local circulation was mostly baroclinic and was dominated by a strong salinity front impinging on the island and large amplitude current reversals with a periodicity of ca. 20 d. During transition times, indicated by a change of the sign of the amplitude of the empirical orthogonal function (EOF), the flow became barotropic. This situation produced strong southward currents followed by the onset of vertical velocity shear. Most of the flow variability occurred in the upper 40 m of the water column, which was also found to be the depth of penetration of the low-salinity lenses. These results indicate that the NBCR structure was retained during both intrusions. Lagrangian trajectories using the MVOA currents were found to be consistent with in situ drifter trajectories, suggesting that the analyzed flow field is representative of the near-shore circulation. Tracking of particles released in the surface layer (0–20 m) from the reef shows a maximum residence time of 18 d indicating the possibility of larval retention within the island-scale flow field. Finally, our results suggest that MVOA, within its limitations, is a powerful tool that can be applied elsewhere to infer circulation and larval transport, even in situations when forcing is unknown. r 2002 Published by Elsevier Science Ltd.

Low-salinity pools at Barbados, West Indies: Their origin, frequency, and variability

A vertical array of conductivity-temperature recorders moored off the west coast of Barbados, West Indies, from May 1996 to November 1997 revealed a heterogeneous and variable salinity pattern punctuated by five pools of low-salinity water (<34.5 practical salinity units (psu)) entering the region. A typical pool extended to 30-m depth and lasted-25 days, although one pool extended to 47 m and lasted 94 days. Water samples taken from a pool in May 1997 have radium 228/226 activity ratios of ∼1, consistent with previous measurements in Barbados of water that originated in the Amazon River mixing zone. The Amazon water likely was translated to Barbados in rings spawned from the North Brazil Current. Analysis of sea height anomaly and residual derived from the TOPEX/Poseidon satellite supports this conclusion and reveals that, contrary to previous studies, rings are shed throughout the year, mostly during spring. The pools of low-salinity water and their associated velocities dramatically changed the already variable flow in our study area. We believe the complex salinity and flow we observed represented the disorganized remnants of North Brazil Current rings that were at or near the ends of their lives. The changes we observed in the velocity and water structure are interesting in their own right as evidence of the Barbados region as a mixing zone and for their influence on recruitment of larval fishes to the reef along the islands west coast.

Bathymetric influences on the lower Chesapeake Bay hydrography

Abstract Observations of salinity and density in the lower Chesapeake Bay are used to describe the bathymetric influence on the transverse hydrographic structure in the area. Current velocity observations of high spatial resolution are also used to relate the flow structure to the hydrographic structure. Tidal flow characteristics in the lower bay are affected by the combination of bathymetry and hydrography. Increased stratification over channels relative to shoals may increase M2 ellipticity with depth over channels but not over shoals. It is found that three consistent hydrographie features can be related to the transverse structure of the longitudinal flow: (1) persistent stratification over channels due to differential tidal advection of density gradients, (2) development of bottom front separating net inflows from net outflows at the region south of Chesapeake Channel, and (3) outflow of low salinity water at the northern end of a lower bay section. Based on these hydrographie features, two basic hydrographic regimes are proposed to exist throughout the year in the lower Chesapeake Bay: (1) a low buoyancy-high mixing energy regime of stratification restricted to channels, a northward monotonical increase in salinity, and a weak bottom front, and (2) a high buoyancy-weak mixing energy regime of stratified conditions everywhere, a large region of northward salinity decrease at the northern half of the section, and a robust bottom front. The dynamics in the transverse direction for the former regime is ageostrophic, and in the latter regime the contribution by geostrophy is approximately 50% as bathymetric influences become less evident.

Estimation of drag coefficient in James River Estuary using tidal velocity data from a vessel--towed ADCP

[1] A phase-matching method is introduced to calculate the bottom drag coefficient in tidal channels with significant lateral variation of depth. The method is based on the fact that the bottom friction in a tidal channel causes tidal velocity to have a phase difference across the channel. The calculation involves a few steps. First, the observed horizontal velocity components are analyzed to obtain the amplitude and phase of the velocity at the major tidal frequency. The phase of the longitudinal velocity is then fitted to a relationship derived from the linearized momentum balance. The drag coefficient is then calculated. This method is applicable to narrow (approximately a few kilometers wide) tidal channels without strong stratification and where the cross-channel variation of surface elevation is negligible compared to tidal amplitude. This analytic approach is easy to implement when appropriate observational data are available. It allows a spatial variation of the drag coefficient, and the resolution is only limited by that of the observations. The method is validated by identical twin experiments and applied to tidal velocity data, obtained in the James River Estuary, using an acoustic Doppler current profiler during spring tides and neap tides in October–November 1996. The obtained bottom drag coefficient ranged from 1.2 � 10 � 3 to 6.9 � 10 � 3 at different positions along two cross-channel transects each 4 km long and 2 to 14 m deep. The maximum drag coefficient is found in the shallowest water near the banks of the estuary, while the minimum values occur between 9 and 12 m in the center of the channel. The friction of the lateral boundary may have contributed to the apparent increase of the bottom friction on the banks. The transverse mean values of the drag coefficient ranges between 2.2 and 2.3 � 10 � 3 for the spring and neap tides, respectively. INDEX TERMS: 4594 Oceanography: Physical: Instruments and techniques; 4203 Oceanography: General: Analytical modeling; 4560 Oceanography: Physical: Surface waves and tides (1255); 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: drag coefficient, tides, analytic model

Impact of North Brazil Current rings on local circulation and coral reef fish recruitment to Barbados, West Indies

Abstract Early observations of the flow environment around the island of Barbados indicated frequent occurrence of strong current reversals associated with surface salinity fronts. Higher resolution spatial and temporal measurements of the flow regime in 1996 and 1997 provided a comprehensive view of the local surface circulation (0-100 m), revealing that external forcing by North Brazil Current (NBC) rings plays a dominant role in the near-field flow variability surrounding the island. NBC ring forcing had comparable effects on the velocity field during both years, indicating that the ring structure was retained while interacting with topography. In the present study, the interaction of NBC rings with coastal flow dynamics and the biological response of the system as measured by recruitment of coral reef fishes is examined. Our observations show that NBC rings can remain quite coherent as they pass the Tobago-Barbados ridge. Further, the flow direction and associated residence time in the vicinity of the island appear to vary depending on the orientation of the rings as they collide with the island. Concurrent biological samples revealed complex responses to the presence of rings in that during some of the events, larval fishes appeared to be rapidly advected away, resulting in a failure of larval settlement, whereas under other conditions larval retention was enhanced and was followed by a settlement pulse. Impingement by a ring did not alter the concentration of water column chlorophyll a (Chl a ), but it did influence the depth of the Chl a maximum. Simultaneous changes were observed in the vertical distribution of fish larvae. Larval fish encountering ring waters exhibited reduced growth rates and longer larval periods, both potentially reducing survival and, ultimately, recruitment success. Overall, results demonstrate that NBC rings interfere with the islandscale flow dynamics around Barbados and interject considerable variability in the local recruitment signal of coral reef fishes.