Sponges are suspension feeders that use flagellated collar-cells (choanocytes) to actively filter a volume of water equivalent to many times their body volume each hour. Flow through sponges is thought to be enhanced by ambient current, which induces a pressure gradient across the sponge wall, but the underlying mechanism is still unknown. Studies of sponge filtration have estimated the energetic cost of pumping to be <1% of its total metabolism implying there is little adaptive value to reducing the cost of pumping by using “passive” flow induced by the ambient current. We quantified the pumping activity and respiration of the glass sponge Aphrocallistes vastus at a 150 m deep reef in situ and in a flow flume; we also modeled the glass sponge filtration system from measurements of the aquiferous system. Excurrent flow from the sponge osculum measured in situ and in the flume were positively correlated (r>0.75) with the ambient current velocity. During short bursts of high ambient current the sponges filtered two-thirds of the total volume of water they processed daily. Our model indicates that the head loss across the sponge collar filter is 10 times higher than previously estimated. The difference is due to the resistance created by a fine protein mesh that lines the collar, which demosponges also have, but was so far overlooked. Applying our model to the in situ measurements indicates that even modest pumping rates require an energetic expenditure of at least 28% of the total in situ respiration. We suggest that due to the high cost of pumping, current-induced flow is highly beneficial but may occur only in thin walled sponges living in high flow environments. Our results call for a new look at the mechanisms underlying current-induced flow and for reevaluation of the cost of biological pumping and its evolutionary role, especially in sponges.

Measurements of velocity and rates of turbulence were made across a fringing coral reef in the Gulf of Aqaba, Red Sea, to determine the effect that the rough topography has on boundary layer mixing and flow dynamics. Observations were made at two fore-reef sites and a nearby sandy slope. The friction velocity, u * , and drag coefficient, C D , were determined directly from turbulent Reynolds stresses measured using acoustic Doppler velocimeters. Values of C D for the coral substrates ranged from 0.009 to 0.015, three to five times greater than over the sandy bottom site. The turbulence dissipation rate, e, was determined by fitting spectra of vertical velocity to the theoretical ''5/3'' law expected for the inertial subrange of turbulence. There was a local balance between production and dissipation of turbulent kinetic energy, signifying that we could estimate u * from either the mean velocity profile, turbulence, or dissipation rate of turbulent kinetic energy. Estimates from all three measures agreed well with mean u * /U o ranging from 0.10 6 0.03 to 0.12 6 0.03, indicating that existing turbulent boundary layer flow theory can be applied to flows over the rough topography of coral reefs. The bottom topography, by enhancing both reef scale and local drag and mixing levels, allows reef biota to more effectively exchange dissolved and particulate matter with oceanic waters.

Seagrass beds alter their hydrodynamic environment by inducing drag on the flow, thereby attenuating wave energy and near-bottom currents. This alters the turbulent structure and shear stresses within and around the seagrass bed that are responsible for the suspension and deposition of sediment. To quantify these interactions, velocity, pressure, and sediment measurements were obtained across a density gradient of an eelgrass Zostera marina bed within a shallow coastal bay (1 to 2 m depth). Eelgrass beds were found to reduce near-bottom mean velocities by 70 to 90%, while wave heights were reduced 45 to 70% compared to an adjacent unvegetated region. Wave orbital velocities within the eelgrass bed were reduced by 20% compared to flow above the bed, primarily acting as a low-pass filter by removing high-frequency wave motion. However, relatively little reduction in wave energy occurred at lower wave frequencies, suggesting that longer period waves were able to effectively penetrate the seagrass meadow.

Rates of mass transfer in coral reefs are governed both by the physical flow environment and the morphology of the coral. Laboratory experiments were conducted to estimate mass transfer in unidirectional and oscillatory flows by measuring the rate of dissolution of gypsum cylinders (clods) placed within the branching structure of three morphologically distinct coral species. Unidirectional flows were varied between 2.9 and 14.1 cm s Ϫ1 and, as expected, mass transfer rates increased with increasing flow and a more open branch spacing. Depending on morphology and flow, mass transfer rates within the interior of the branching structure were 50 to 75% of that measured outside the coral in free-stream conditions. Oscillatory conditions showed relative mass transfer rates 1.6 to 2.9 times greater than equivalent unidirectional currents. This ratio increased with increasing wave frequency, likely due to the corresponding decrease in the diffusive boundary layer thickness. The ratio also increased with a greater compactness in branch spacing, with mass transfer rates within the coral structure up to 130% of free-stream conditions. We used planar laser-induced fluorescence imaging to study the instantaneous structure of mass advection through the coral. Oscillatory flow acts as a dominant forcing mechanism to generate water motion within the coral structure at levels not attainable with comparable unidirectional currents.Coral reefs exist under a wide range of physical environments and flow conditions, from high-energy environments where they are exposed to wave action as well as wave breaking, to low-energy environments where they are exposed to weak currents driven primarily by wind or tides (e.g., Hamner and Wolanski 1988;Kench 1998). Given this variety of physical environments, it is not surprising that studies have shown a link between variations in flow and habitat to coral growth patterns and morphologic zonation (Graus and Macintyre 1989;Sebens and Done 1992). Changes in growth form can occur in response to flow exposure (Lesser et al. 1994), with transformations from compact branching structures under exposed conditions, to a thinbranching shape in sheltered conditions (Kaandorp 1999). These morphologic modifications, resulting in changes to relative branch size, spacing, and branching pattern, can in turn alter the structure of flow itself, both around and through colonies (Chamberlain and Graus 1975;Sebens et al. 1997). Therefore, there is an apparent feedback between water flow patterns and coral morphology, which may not only play a critical role in coral survival, affecting colony overturning and breaking (Done 1982), but also have a sig-1 Present address: Department of Integrative Biology, University of California, Berkeley, California 94720. AcknowledgmentsWe thank Marlin Atkinson and Jim Falter at the Hawaii Institute of Marine Biology for supplying the P. compressa coral skeletons and Bob Brown for the design and construction of the resonant wavemaker.

In this paper hydrographic observations made over a fringing coral reef at the northern end of the Gulf of Aqaba near Eilat, Israel, are discussed. These data show exchange flows driven by the onshore-offshore temperature gradients that develop because shallow regions near shore experience larger temperature changes than do deeper regions offshore when subjected to the same rate of heating or cooling. Under heating conditions, the resulting vertically sheared exchange flow is offshore at the surface and onshore at depth, whereas when cooling dominates, the pattern is reversed. For summer conditions, heating and cooling are both important and a diurnally reversing exchange flow is observed. During winter conditions, heating occupies a relatively small fraction of the day, and only the cooling flow is observed. When scaled by ⌬V, the observed profiles of the cross-shore during cooling velocity collapse onto a single curve. The value of ⌬V depends on the convective velocity scale u f and the bottom slope ␤ through the inertial scaling, ⌬V ϳ ␤ Ϫ1/3 u f first proposed by Phillips in the 1960s as a model of buoyancy-driven flow in the Red Sea. However, it is found that turbulent stresses associated with the longshore tidal flows and unsteadiness due to the periodic nature of the buoyancy forcing can act to weaken the sheared exchange flow. Nonetheless, the measured exchange flow transport agrees well with previous field and laboratory work. The paper is concluded by noting that the "thermal siphon" observed on the Eilat reef may be a relatively generic feature of the nearshore physical oceanography of reefs and coastal oceans in general.