Donald B. Olson

Inputs, losses and transformations of nitrogen and phosphorus in the pelagic North Atlantic Ocean

The North Atlantic Ocean receives the largest allochthonous supplies of nitrogen of any ocean basin because of the close proximity of industrialized nations. In this paper, we describe the major standing stocks, fluxes and transformations of nitrogen (N) and phosphorus (P) in the pelagic regions of the North Atlantic, as one part of a larger effort to understand the entire N and P budgets in the North Atlantic Ocean, its watersheds and overlying atmosphere. The primary focus is on nitrogen, however, we consider both nitrogen and phosphorus because of the close inter-relationship between the N and P cycles in the ocean. The oceanic standing stocks of N and P are orders of magnitude larger than the annual amount transported off continents or deposited from the atmosphere. Atmospheric deposition can have an impact on oceanic nitrogen cycling at locations near the coasts where atmospheric sources are large, or in the centers of the highly stratified gyres where little nitrate is supplied to the surface by vertical mixing of the ocean. All of the reactive nitrogen transported to the coasts in rivers is denitrified or buried in the estuaries or on the continental shelves and an oceanic source of nitrate of 0.7–0.95 × 1012 moles NO3−1 y−1 is required to supply the remainder of the shelf denitrification (Nixon et al., this volume). The horizontal fluxes of nitrate caused by the ocean circulation are both large and uncertain. Even the sign of the transport across the equator is uncertain and this precludes a conclusion on whether the North Atlantic Ocean as a whole is a net source or sink of nitrate. We identify a source of nitrate of 3.7–6.4 × 1012 moles NO3− y−1 within the main thermocline of the Sargasso Sea that we infer is caused by nitrogen fixation. This nitrate source may explain the nitrate divergence observed by Rintoul & Wunsch (1991) in the mid-latitude gyre. The magnitude of nitrogen fixation inferred from this nitrate source would exceed previous estimates of global nitrogen fixation. Nitrogen fixation requires substantial quantities of iron as a micro-nutrient and the calculated iron requirement is comparable to the rates supplied by the deposition of iron associated with Saharan dust. Interannual variability in dust inputs is large and could cause comparable signals in the nitrogen fixation rate. The balance of the fluxes across the basin boundaries suggest that the total stocks of nitrate and phosphate in the North Atlantic may be increasing on time-scales of centuries. Some of the imbalance is related to the inferred nitrogen fixation in the gyre and the atmospheric deposition of nitrogen, both of which may be influenced by human activities. However, the fluxes of dissolved organic nutrients are almost completely unknown and they have the potential to alter our perception of the overall mass balance of the North Atlantic Ocean.

Seascape genetics: a coupled oceanographic-genetic model predicts population structure of Caribbean corals.

Population genetics is a powerful tool for measuring important larval connections between marine populations [1-4]. Similarly, oceanographic models based on environmental data can simulate particle movements in ocean currents and make quantitative estimates of larval connections between populations possible [5-9]. However, these two powerful approaches have remained disconnected because no general models currently provide a means of directly comparing dispersal predictions with empirical genetic data (except, see [10]). In addition, previous genetic models have considered relatively simple dispersal scenarios that are often unrealistic for marine larvae [11-15], and recent landscape genetic models have yet to be applied in a marine context [16-20]. We have developed a genetic model that uses connectivity estimates from oceanographic models to predict genetic patterns resulting from larval dispersal in a Caribbean coral. We then compare the predictions to empirical data for threatened staghorn corals. Our coupled oceanographic-genetic model predicts many of the patterns observed in this and other empirical datasets; such patterns include the isolation of the Bahamas and an east-west divergence near Puerto Rico [3, 21-23]. This new approach provides both a valuable tool for predicting genetic structure in marine populations and a means of explicitly testing these predictions with empirical data.

Influence of monsoonally-forced Ekman dynamics upon surface layer depth and plankton biomass distribution in the Arabian Sea

Abstract Oceanic surface layer properties in the central Arabian Sea at the height of the northeast 1986, 1987 and southwest 1987 monsoons are described. Data from two research cruises on the R.R.S. Charles Darwin are combined with Comprehensive Ocean—Atmosphere Data Set (COADS) winds and a simple model to consider the influence of the monsoons on the surface layers of the Arabian Sea. In December 1986 (NE monsoon) the top of the thermocline was fairly uniform with a mean depth of approximately 60 m. In contrast, this depth varied with latitude in the SW monsoon, with the shallowest depths occurring under the region of maximum winds and the maximum depths to the south of the monsoon jet. This situation is tied to the Ekman flow which leads to open ocean upwelling to the north of the wind maximum, thus suppressing the development of the mixed layer. The thermocline to the south is deepened by a combination of Ekman pumping and advection of dense fluid from the north. A simple two-dimensional model incorporating a non-penetrative mixed layer and a slab Ekman layer flow is used to examine the factors involved in the evolution of the surface layers during the monsoons. The combination of upwelling and surface layer deepening leads to latitudinal gradients in phytoplankton biomass. The major effect in the SW monsoon is the upwelling induced by Ekman dynamics and a consequent shoaling of the nitracline in the northern Arabian Sea. Vertical mixing and the associated deepening of the surface layer dominate phytoplankton distributions in the NE monsoons.

Maintenance of the low-oxygen layer in the central Arabian Sea

Abstract An intermediate depth layer, approximately 1 km thick, in the northwestern Indian Ocean contains essentially no detectable dissolved oxygen. Previous suggestions for primary causes of this feature have been: (a) very slow movement within the layer, allowing a long time for organic decomposition to consume the oxygen; (b) very large local consumption rates, resulting from enormous productivity in the surface layer; or (c) low oxygen concentrations in the waters entering the layer from the south, due to their long transit from their sea-surface sources. Observations reported here of a transient anthropogenic trace gas, trichlorofluoromethane (F-11 or freon 11), however, demonstrate that the residence time for water in the low-oxygen layer is not espciallt long, about 10 years. Concurrent summertime measurements of surface productivity, while high, preclude an exceptional mean consumption rate at depth. An oxygen budget for the layer supports the idea that the near-zero concentration is maintained by moderate consumption applied to waters with initially low oxygen concentration that pass through the layer at moderate speed.

A four-component ecosystem model of biological activity in the Arabian Sea

Abstract A coupled, physical-biological model is used to study the processes that determine the annual cycle of biological activity in the Arabian Sea. The physical model is a 2 1 2 - layer system with a surface mixed layer imbedded in the upper layer, and fluid is allowed to move between layers via entrainment, detrainment and mixing processes. The biological model consists of a set of advective-diffusive equations in each layer that determine the nitrogen concentrations in four compartments: nutrients, phytoplankton, zooplankton and detritus. Coupling is provided by the horizontal-velocity, layer-thickness, entrainment and detrainment fields from the physical solution. Surface forcing fields (such as wind stress and photosynthetically active radiation) are derived from monthly climatological data, and the source of nitrogen for the system is upward diffusion of nutrients from the deep ocean into the lower layer. Our main-run solution compares favorably with observed physical and biological fields; in particular, it is able to simulate all the prominent phytoplankton blooms visible in the CZCS data. Three bloom types develop in response to the physical processes of upwelling, detrainment and entrainment. Upwelling blooms are strong, long-lasting events that continue as long as the upwelling persists. They occur during the Southwest Monsoon off Somalia, Oman and India as a result of coastal alongshore winds, and at the mouth of the Gulf of Aden through Ekman pumping. Detrainment blooms are intense, short-lived events that develop when the mixed layer thins abruptly, thereby quickly increasing the depth-averaged light intensity available for phytoplankton growth. They occur during the fall in the central Arabian Sea, and during the spring throughout most of the basin. In contrast to the other bloom types, entrainment blooms are weak because entrainment steadily thickens the mixed layer, which in turn decreases the depth-averaged light intensity. There is an entrainment bloom in the central Arabian Sea during June in the solution, but it is not apparent in the CZCS data. Bloom dynamics are isolated in a suite of diagnostic calculations and test solutions. Some results from these analyses are the following. Entrainment is the primary nutrient source for the offshore bloom in the central Arabian Sea, but advection and recycling also contribute. The ultimate cause for the decay of the solutions spring (and fall) blooms is nutrient deprivation, but their rapid initial decay results from grazing and self shading. Zooplankton grazing is always an essential process, limiting phytoplankton concentrations during both bloom and oligotrophic periods. Detrital remineralization is also important: in a test solution without remineralization, nutrient levels drop markedly in every layer of the model and all blooms are severely weakened. Senescence, however, has little effect: in a test solution without senescence, its lack is almost completely compensated for by increased grazing. Finally, the models detrainment blooms are too brief and intense in comparison to the CZCS data; this difference cannot be removed by altering biological parameters, which suggests that phytoplankton growth in the model is more sensitive to mixed-layer thickness than it is in the real ocean.

Dynamics of the Brazil-Malvinas Confluence based on inverted echo sounders and altimetry

We use data from Geosat altimeter and from 10 inverted echo sounder (IES) moorings deployed in the SW Atlantic Ocean off the Argentine continental shelf to investigate several aspects of the dynamics of the upper layer in the Brazil-Malvinas Confluence region. We use the altimeter data to estimate the sea height anomalies at each IES location and use the IES data to compute the upper layer thickness, taken in this work to go to the depth of the 8°C isotherm. We first discuss the sea height and upper layer thickness variations caused by the passage of the Brazil Current, Malvinas Current, and warm anticyclonic and cold cyclonic eddies. We introduce a two-layer model in which we decompose the sea height into its baroclinic and barotropic contributions. We then propose a method to monitor the thickness of the upper layer and the barotropic and baroclinic transports as a function of the sea height anomalies and the statistics of the upper layer thickness and reduced gravity for the region. We compute the reduced gravity values from the slope of a linear fit between the sea height anomalies and the upper layer thicknesses. We estimate the reduced gravity values for this region to range from 0.005 to 0.011 m s−2. We also estimate the mean barotropic sea height difference using two methods: conservation of mass and conservation of potential vorticity. Finally, we compute the time series for the baroclinic and barotropic transports during the Geosat Exact Repeal Mission time period. Our results suggest that the mean baroclinic transport in the upper layer decreases from 12 Sv at around 35°S to 7 Sv at 37°S. Our results also indicate that there is a significant barotropic contribution to the upper layer transport in the confluence region.

Modeling the effect of nitrogen fixation on carbon and nitrogen fluxes at BATS

Recent geochemical estimates of N2-fixation in the North Atlantic ocean indicate rates that are significantly higher than those derived from direct observations. In this paper different N2-fixation rate scenarios are explored using a one-dimensional, biogeochemical model that includes an explicit representation of Trichodesmium. This model reproduces most of the observed interannual variability in phytoplankton production and generates seasonal Trichodesmium biomass and N2-fixation cycles similar to those observed at BATS. Two solutions are presented, one where the N2-fixation rate is increased enough to reproduce the observed summertime drawdown of DIC, and a second where it is tuned to reproduce the observed sediment trap fluxes. The high N2-fixation solution reproduces the seasonal and interannual variability in DIC concentrations quite accurately and generates N2-fixation rates that agree with direct rate measurements from 1990 and recent geochemical estimates. However, this solution generates export fluxes that are more than 4 times higher than those observed, and predicts the development of DON and DOC anomalies in late summer/early fall that have not been observed. In contrast, the low N2-fixation solution generates trap fluxes that are approximately correct, but overestimates the summertime DIC concentrations by 20–View the MathML source. Both solutions indicate that there is significant interannual variability in N2-fixation at BATS and that the rates were much lower in 1995–1996 than in the previous six years. It is suggested that this variability is linked to decadal-scale fluctuations in the North Atlantic climate.

Agulhas ring dynamics from TOPEX/POSEIDON satellite altimeter data

The transfer of warm water from the Indian Ocean into the South Atlantic subtropical gyre takes place in the form of rings and filaments formed when the Agulhas Current retroflects south of Africa between IS and 25E. A survey of the rings formed from September 1992 until December 1995 in the Retroflection region was carried out using TOPEX/POSEIDON altimeter data. A two-layer model was used to estimate the upper layer thickness from the altimeter-derived sea-surface height anomaly data. An objective analysis scheme was used to construct a map of upper layer thickness every ten days. Seventeen rings and their trajectories were identified using these maps. The shedding of rings from the Agulhas Current was neither continuous nor periodic, and for long periods there is no formation of rings. Several rings remained in the region for more than a year and, at any given time, 2 to 6 rings coexisted in the region east of the Walvis Ridge. The results showed that the number of rings translating simultaneously in this region is larger during the first half of each year. The upper layer transport of the Agulhas Current in the Retroflection region was computed and a close association between high variations in transport and ring shedding was found. Rings translated WNW at translation speeds ranging from 5 to 16 km day-l following formation. The values of available potential energy computed for the rings place them among the most energetic rings observed in the world oceans, with values of up to 70 X lOIs J. Transport computations indicate that each ring contributes in the average approximately 1 Sv of Agulhas Current waters to the Benguela Current.

The Physical Oceanography of Two Rings Observed by the Cyclonic Ring Experiment. Part II: Dynamics

Abstract Data from the 1977 Cyclonic Ring Experiment is used to examine the, density, velocity, vorticity and energy distributions in a Gulf Stream cyclonic ring. A time series on ring BOB provides information on the temporal changes in these parameters. The rings are shown to have an interior in near-solid-body rotation surrounded by a frontal jet exhibiting a maximum in potential vorticity. There is evidence that the baroclinic structure of the rings extends to the bottom and that there is significant energy in vertical modes higher than the first baroclinic mode. Calculations of available potential energy (APE) and kinetic energy are presented along with the time rates of change in the APE. The APE is partitioned into the energy in the mean ring and a set of perturbations about the mean. The temporal variations in the perturbation APE are large enough to be important to the adjustment and decay of mean APE in the ring. Ring spindown is discussed in terms of the observations and several ring models prop...

Observations of seasonal exchange through the Straits of Hormuz and the inferred heat and freshwater budgets of the Persian Gulf

The exchange between the Persian (Arabian) Gulf and the Indian Ocean is investigated using hydrographic and moored acoustic Doppler current profiler data from the Straits of Hormuz during the period December 1996 to March 1998. The moored time series records show a relatively steady deep outflow through the strait from 40 m to the bottom with a mean speed of approximately 20 cm/s. A variable flow is found in the upper layer with frequent reversals on timescales of several days to weeks. The annual mean flow in the near-surface layer is found to be northeastward (out of the Persian Gulf) in the southern part of the strait, suggesting a mean horizontal exchange with the Indian Ocean that is superimposed on the vertical overturning exchange driven by evaporation over the gulf. The salinity of the deep outflow varies from 39.3 to 40.8 psu with highest outflow salinities occurring in the winter months (December–March). The annual mean deep outflow through the strait is estimated to be 0.15 ± 0.03 Sv. Calculation of the associated heat and freshwater fluxes through the strait yields estimates for the annual heat loss over the surface of the gulf of ?7 ± 4 W/m2 and an annual water loss (E-P-R) of 1.68 ± 0.39 m/yr. These values are shown to be in relatively good agreement with climatological surface fluxes derived from the Southampton Oceanography Centre global flux climatology after known regional biases in the radiative budget are taken into account.