Kevin E. Kohler

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.

Influence of precipitation minus evaporation and Bay of Bengal rivers on dynamics, thermodynamics, and mixed layer physics in the upper Indian Ocean

A 4½-layer model with active thermodynamics and mixed layer physics is used to examine how salinity distributions forced by precipitation P minus evaporation e and by river runoff in the Bay of Bengal affect dynamics, thermodynamics, and mixed layer physics in the upper Indian Ocean. Each of the four active layers represents a distinct water mass type: the surface mixed layer, the seasonal thermocline (barrier layer in the tropics), the thermocline, and upper intermediate water. Waters are allowed to transfer between layers by interfacial velocities w1, w2, and w3. Velocity w1 parameterizes entrainment and detrainment from the surface mixed layer, and it is determined largely by Kraus and Turner [1967] physics. Velocity w2 is primarily a parameterization of subduction. In regions where precipitation is strong enough for P − e > 0, forcing by P − e thins the surface mixed layer (layer 1) because of decreased entrainment, and thus thickens the seasonal thermocline (layer 2, a barrier layer). Additionally, surface currents generally strengthen, T2 warms considerably, and sea surface temperature (SST) increases somewhat, resulting in temperature inversions at some locations in the southern bay and eastern equatorial ocean. This forcing also causes large temperature changes in the thermocline (layer 3), primarily because of heating or cooling by anomalous subduction. During the Southwest Monsoon, forcing by inflow from Bay of Bengal rivers increases SST by 0.5°–1°C along the northeast coast of India. This is because coastal Kelvin waves driven by the Ganges-Brahmaputra River inflow suppress coastal upwelling there. During the Northeast Monsoon, fresh river water is carried southward by the East India Coastal Current (EICC), raising sea level along the coast and strengthening the EICC by 10 cm s−1. The river water decreases entrainment around the perimeter of the bay during winter, thereby thinning the surface mixed layer, increasing T2 and resulting in temperature inversions in the northwestern bay. River inflow also causes significant temperature anomalies in layer 3 by affecting subduction.

Influences of diurnal and intraseasonal forcing on mixed‐layer and biological variability in the central Arabian Sea

A three-dimensional, physical-biological model of the Indian Ocean is used to study the influences of diurnal and intraseasonal forcing on mixed-layer and biological variability in the central Arabian Sea, where a mooring was deployed and maintained from October 1994 to October 1995 by the Woods Hole Oceanographic Institution Upper Ocean Processes group. The physical model consists of four active layers overlying an inert deep ocean, namely, a surface mixed layer of thickness h1, diurnal thermocline layer, seasonal thermocline, and main thermocline. The biological model consists of a set of advective-diffusive equations in each layer that determine nitrogen concentrations in four compartments: nutrients, phytoplankton, zooplankton, and detritus. Both monthly climatological and “daily” fields are used to force solutions, the latter being a blend of daily-averaged fields measured at the mooring site and other products that include intraseasonal forcing. Diurnal forcing is included by allowing the incoming solar radiation to have a daily cycle. In solutions forced by climatological fields, h1 thickens steadily throughout both monsoons. When h1 detrains at their ends, short-lived, intense blooms develop (the models spring and fall blooms) owing to the increase in depth-averaged light intensity sensed by the phytoplankton in layer 1. In solutions forced by daily fields, h1 thins in a series of events associated with monsoon break periods. As a result, the spring and fall blooms are split into a series of detrainment blooms, broadening them considerably. Diurnal forcing alters the mixed-layer and biological responses, among other things, by lengthening the time that h1 is thick during the northeast monsoon, by strengthening the spring and fall blooms and delaying them by 3 weeks, and by intensifying phytoplankton levels during intermonsoon periods. Solutions are compared with the mixed-layer thickness, phytoplankton biomass, and phytoplankton production fields estimated from mooring observations. The solution driven by daily fields with diurnal forcing reproduces the observed fields most faithfully.

The Statistics of Natural Shapes in Modern Coral Reef Landscapes

Spatial heterogeneity is a fundamental characteristic of modern and ancient depositional settings, and the scaling of many carbonate environments has been shown to follow power function distributions. The difficulty in obtaining information on the horizontal persistence of sedimentary lithotopes at the basin scale has, however, hampered evaluation of this fact over larger geographic areas. In recent years, large‐scale maps of reef facies derived from remotely sensed data have become widely available, allowing for an analysis of reef‐scale map products from 26 sites spread through four reef provinces, covering >7000 km2 of shallow‐water habitat in the U.S. territorial Pacific. For each site, facies maps were decomposed to polygons describing the perimeter of patches of differing sedimentologic/benthic character. A suite of geospatial metrics quantifying unit shape, fractal dimension, and frequency‐area relations was applied to investigate the intra‐ and intersite variability. The spatial architecture of these reef sites displays robust fractal properties over an extended range of scales with remarkable consistency between provinces. These results indicate the possibility of extrapolating information from large to small scales in various depositional environments.

A Tale of Germs, Storms, and Bombs: Geomorphology and Coral Assemblage Structure at Vieques (Puerto Rico) Compared to St. Croix (U.S. Virgin Islands)

Abstract The former U.S. Navy range at Vieques Island (Puerto Rico, United States) is now the largest national wildlife refuge in the Caribbean. We investigated the geomorphology and benthic assemblage structure to understand the status of the coral reefs. Coral assemblages were quantified at 24 sites at Vieques and at 6 sites at St. Croix, U.S. Virgin Islands. These sites were chosen to represent the major zones of reef geomorphology. Sites consisted of two or three 21-m-long photo-quadrate belt transects. The results revealed surprisingly little differentiation in the coral assemblages within and between reefs of comparable geomorphological and oceanographic setting at Vieques and St. Croix. At Vieques, the Acropora palmata zone was almost completely lost, and it was severely reduced at St. Croix, presumably primarily due to diseases and hurricane impacts since the 1970s. Subtle, but nonsignificant, differences with respect to the nature of the shelf margin (north adjacent to the bank, south adjacent to the open sea) and depth zone were observed at Vieques. At St. Croix, benthic assemblages differed more between depth zones but not between north and south. Effects of natural disturbances were severe at Vieques, outweighing impacts of past military activity—which were present but not quantitatively discernible at our scale of sampling. Germs and storms, rather than bombs (and associated naval activities), primarily seem to have taken the worst toll on corals at both Vieques and St. Croix.