Vertical mixing in a shallow tropical reservoir

Limnology - This paper presents observations of diurnal cycles of stratification and vertical mixing in Kranji Reservoir, a shallow tropical reservoir with an average depth of 6.7 m...     

Flow through a rough,shallow reef

This paper presents a simple model of wave-driven flow through a coral reef that is characterized by a shallow, wide reef crest and a deeper, but frictional lagoon. Assuming that the dominant momentum balances are between quadratic drag and barotropic pressure gradients, along-lagoon and cross-reef flows are coupled through continuity and through a setup of the water surface in the lagoon that varies in the along-reef direction. Scaling of the governing equations shows that this flow is governed by a single parameter P that expresses the competing effects of cross-reef and along-lagoon drag. When P < 1, the cross-reef flow is nearly constant, whereas when P > 1, only that portion of the reef closest to the pass through the reef crest through which fluid exists the lagoon supports cross-reef flows.     

Near-bottom depletion of zooplankton over a coral reef II: relationships with zooplankton swimming ability

Intense predation by corals and associated fauna can generate vertical gradients of their zooplankton prey. The goal of this study was to characterize the small-scale distribution of zooplankton above the coral reef of Eilat, Israel. Four vertical arrays, each consisting five underwater pumps attached 0.5–3 m apart on a taut mooring, were deployed for 10 days at two sites over the reef slope. A distinct layer of depleted zooplankton was repeatedly found in the 1.5 m high benthic boundary layer. The gradient was sharpest for strong swimmers (copepods and polychaeta), intermediate for weak swimmers (nauplii, mollusks and appendicularia), and lacking for passive taxa (eggs and foraminifera). Spatio-temporal changes in the abundance of copepods and polychaetes were highly correlated, more so in the water aloft (Pearson r>0.9) than near the bottom (r>0.8). The spatio-temporal correlations between weak swimmers and between passive taxa were much weaker (r<0.8 and r<0.4, respectively). Flow and shear stress did not affect the distribution patterns. The correspondence between the zooplankton distribution and their swimming ability indicates that bottom avoidance contribute to the formation of depleted layer over the reef.     

Short-term coherency between gross primary production and community respiration in an algal-dominated reef flat

Rates of net community carbon production (mmol C m⁻² h⁻¹) were measured continuously in an algal-dominated reef flat community on the Kaneohe Bay barrier reef, Hawaii, for 12 days at the end of October 2006. The weather became increasingly cloudy during the last 5 days of measurements, resulting in a sevenfold decline in daily incident light (28–4 Ein m⁻² d⁻¹). In response, gross primary production (P) for the reef flat community also decreased sevenfold, varying linearly with light (r ² = 0.92, n = 12). Community respiration (R) decreased fivefold over this same period and was highly correlated with changes in P (r ² = 0.84, n = 12). We reason that this short-term coherence between P and R indicates that most of the carbon fixed during this period was rapidly metabolized via plant respiration. We further conclude that the dominance of autotrophic respiration under general conditions of nutrient-limited growth can explain much of the balance between P and R that is commonly observed in shallow reef communities.     

On the effects of topography on wind and the generation of currents in a large multi-basin lake

The wind field over a lake surface is the key element in driving the exchange of momentum and energy at the free surface. It is rarely uniform, having a considerable degree of spatial variability, both on a synoptic and a local scale. Topographic features are one of the most common contributors to this variability. The aerodynamic effects of topography on the windfield, and its influence on the circulation patterns and interbasin exchange rates in a large multi-basin lake are documented. The complex landscape that surrounds this lake is dominated by the presence of a mountain peak rising over 900m above the lake level. The circulation patterns in those basins of the lake located in the leeward side of the mountain reflect the spatial patterns of the wind field generated by the surrounding topography. The spatial variations in the wind field are shown to significantly alter the residence times of each basin and the exchange rates between basins, isolating one from the other two basins of the lake. The inter-basin exchange rates determined with wind field variability are consistent with annual contaminant loadings estimated for each of the three basins, which suggests a close link between chemical and hydrodynamic behavior.