Hydrodynamics of intertidal oyster reefs: The influence of boundary layer flow processes on sediment and oxygen exchange

Abstract

Lay Abstract The Eastern oyster, Crassostrea virginica, often forms intertidal reefs composed of dense assemblages of oysters. Because of their suspension feeding, oysters clear large quantities of particulate matter from the water column, removing not only phytoplankton but also suspended sediment. In addition, because of their metabolic requirements, oysters can utilize substantial quantities of oxygen. An intertidal oyster reef was monitored to quantify processes affecting flow, suspended sediment deposition and erosion, and the exchange of oxygen to and from the reef. At the center of the reef, the flux of oxygen was measured using an eddy‐correlation technique, while sediment uptake was computed from the difference between upstream and downstream suspended sediment concentrations across the reef. While the reef was submerged, oxygen uptake increased linearly with velocity, ranging between 100 and 600 mmol m − 2 d − 1. Sediment deposition to the reef also increased linearly for velocities between 0 and 10 cm s − 1. However, for velocities >15 cm s − 1, sediment uptake by the reef decreased, and at flows >25 cm s − 1 there was net erosion from the reef due to the resuspension of sediment. Overall, results show that oysters have a net positive effect on water clarity and that water velocities exert a strong influence on the exchange of oxygen and sediment to and from the reef surface.

Benthic O2 uptake of two cold-water coral communities estimated with the non-invasive eddy correlation technique

“…Due to the natural structural complexity of CWC, we observed z 0 values (3.4 and 3.0 cm for Mingulay and Stjernsund, respectively) that were much higher than those typical for rough gravel or cobble beds (< 0.5 cm; Reidenbach et al 2010) and comparable to that of oyster beds (2.7 cm; Reidenbach et al 2013). As increased z 0 results in increased turbulent mixing, the estimated footprint areas in the present study were relatively small.…”

“…Yet, one significant advantage of the EC technique is that it can be deployed on hard substrates, where chamber and microprofile measurements are easily compromised or fail entirely. The EC technique has been successfully applied to structurally complex, hard benthic substrates such as tropical coral reefs (Long et al 2013), high-latitude rocky embayments (Glud et al 2010) and oyster beds (Reidenbach et al 2013). Since EC O 2 exchange measurements are obtained under natural hydrodynamics and characterize large areas of the seabed, the measurements integrate much of the site-specific spatial and temporal variability and provide a robust measure of benthic O 2 exchange of heterogeneous benthic communities (Rheuban & Berg 2013).…”

Seasonal rates of benthic primary production in a Greenland fjord measured by aquatic eddy correlation

“…The time required for changes in the benthic O 2 exchange rate to be registered at the EC measuring height may be significantly longer than the time window over which the EC fluxes are analyzed, becoming larger as the measurement height increases and the z 0 decreases (Rheuban and Berg 2013). The range of z 0 values (0.001-0.017 m) and C D values (0.004-0.023) we estimated indicate rough bed geometry typical of sands, gravels, and cobbles (Reidenbach et al 2010). A rough bed geometry results in vigorous turbulent mixing that rapidly transports the signal from the seabed to the EC measurement point.…”

“…To date, coastal applications of the EC technique constitute a limited but growing database. EC measurements on hard-bottom benthic surfaces (Glud et al 2010), seagrass beds (Hume et al 2011), permeable sediments , sea ice (Long et al 2012), coral reefs (Long et al 2013), and oyster beds (Reidenbach et al 2013) document the validity of this method in environments where benthic chamber and microprofile measurements would be compromised.…”

Mussels blow rings: Jet behavior affects local mixing

“…By removing suspended material from the water column, suspension feeders also play important roles in regulating water clarity, dissolved oxygen concentrations, and nutrient cycling (North et al, 2012;Cha et al, 2013;Greene et al, 2015; see references in Nalepa and Schloesser, 2014). Although they represent an important link between benthic and pelagic habitats (Ackerman et al, 2001;Hecky et al, 2004), the activity of many bottom dwellers may be limited by low mixing rates in the benthic boundary layer (BBL) (Frechette et al, 1989;Ackerman et al, 2001;Reidenbach et al, 2013). The exhalant jets from active feeders contain momentum (O'Riordan et al, 1995), and some work indicates that these types of jets increase mixing of nearbottom waters (O'Riordan et al, 1995;Larsen and Riisgård, 1997;Lassen et al, 2006;van Duren et al, 2006), whereas others suggest that they do not (Smaal et al, 1986;Ertman and Jumars, 1988).…”