Appendix A. Natural history, including ecological and environmental interactions of Atrina zelandica.
Atrina occur in both muddy and sandy environments, in water depths from 2 to 50 m. Atrina are often patchily distributed on the 10- to 100-m scale and patches are composed exclusively of similar-sized individuals (Hewitt et al. 2002). Adult individuals typically protrude 0.10-0.20 m above the seabed and do not display a preferred orientation relative to tidal flows (Green et al. 1998, Fig. A1). Previous studies have demonstrated that Atrina patches influence the structure of benthic communities (Warwick et al. 1997, Cummings et al. 1998, 2001) and that the density, size and spatial structure of Atrina, sediment type (grain size and organic content), flow velocity, and suspended sediment concentration (SSC) can modify the effect of Atrina on the surrounding macrofaunal communities (Norkko et al. 2001, Thrush et al. 2001, Hewitt et al. 2002). Atrina patches influence boundary-layer flows at both the 100-m (Green et al. 1998) and 1-m scales (Nikora et al. 2002). Furthermore, Atrina, while having low feeding rates (6·10-4 m3/sec), have the ability to reject unwanted matter as pseudofaeces and their physiological condition is negatively affected by elevated SSC (Ellis et al. 2002, Hewitt and Pilditch, 2004, Miller et al. 2002).
Atrina are an ideal subject for the development of a patch-scale model, since individuals are comparatively long-lived (projected to live 30 years and reach maturity after three years) and immobile. Furthermore, patches consist of one size class, which is indicative of the settlement of a single cohort (Hewitt et al., 2002; Fig. A2). Despite extensive observations, we have only once found a discrete dense patch of juveniles. Observations at a number of locations around New Zealand indicate that juveniles do not recruit into existing patches. Our observations on the recruitment and life history of Atrina are similar to those reported for the closely related Pinna bicolor and Pinna noblis (Butler et al. 1993). Once established, individuals grow and the patches evolve over many years unless the Atrina are subjected to high levels of stress, disturbance or mortality. These characteristics enable us to ignore recruitment when dealing with the dynamics of a single patch; hence, recruitment is not represented in the model.
Observations have suggested that the distribution of Atrina in the coastal environment is limited by SSC, and this has been supported by field transplant and laboratory experiments (Ellis et al. 2002). Changes in Atrina physiological and biochemical condition indices have been detected after only 3 days when SSC exceeds 80 mg/L, which occurs frequently during rain and wind storms (Ellis et al. 2002). These biological characteristics enable us to consider how the size and density of Atrina within a patch may interact with both anthropogenic stress (SSC) and environmental conditions (depth and tidal velocity), mediated through changes in the condition and mortality of individuals within a patch.
The presence of organisms and biogenic structures on the seafloor complicates boundary-layer flows because of the density, and height of these individual roughness elements. Vogel (1994) identified three distinct types of flow (independent, interacting and skimming) as a function of the ratio between spacing and height of elements protruding from the bed. If the spacing of individual elements is large in comparison to the height, flow is essentially unaffected (independent flow), and as elements become more closely packed, turbulence levels close to the bed increase (interacting flow). If the spacing further decreases, flow does not penetrate in between roughness elements and turbulence levels close to the bed substantially decrease (skimming flow). The development of skimming flow has been observed in a variety of natural systems, including canopies (Poggi et al. 2004), algal mats (Romano et al. 2003), clam beds (Crimaldi et al. 2002), and Atrina beds (Green et al. 1998). Previous notions of skimming flow developing when roughness elements cover up to 12% of the patch surface (Nowell and Church 1979, Eckman et al. 1981) should be considered as purely indicative, as the height of the roughness elements was not accounted for. For Atrina, Green et al. (1998) investigated the effect of density on the drag coefficient and reported “anomalous” low values over dense patches that were attributed to skimming flow. These field measurements corroborate other predictions and laboratory observations that suggest suspension-feeder density is likely to control and determine hydrodynamics at the patch scale (Nowell and Jumars 1984, Johnson 1990, Friedrichs et al. 2000, Friedrichs 2004). In turn, skimming flow has obvious implications for sediment transport, resuspension and food resources available to suspension-feeders. At smaller space and shorter time scales, feeding currents generated by suspension-feeding bivalves can also affect flows (Eckman 1985, Frechette et al. 1989, Monismith et al. 1990 , O’Riordan et al. 1995, Norkko et al. 2001, Jonsson et al. 2005). For Atrina such flow perturbations appear to be small and have limited influence on the overall patch dynamics (Nikora et al. 2002).
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FIG. A1. Dense patch of Atrina (photo by S. Thrush).
FIG. A2. Variation in the size of Atrina (mean width ± SD) recorded during long-term monitoring of a site in 6 m water depth in Mahurangi Harbour. Data derived from 10 randomly located 0.25m2 quadrates sampled in October and February each year (Cummings et al. 2003).