Coastal oceanography sets the pace of rocky intertidal community dynamics

Abstract

The structure of ecological communities reflects a tension among forces that alter populations. Marine ecologists previously emphasized control by locally operating forces (predation, competition, and disturbance), but newer studies suggest that inputs from large-scale oceanographically modulated subsidies (nutrients, particulates, and propagules) can strongly influence community structure and dynamics. On New Zealand rocky shores, the magnitude of such subsidies differs profoundly between contrasting oceanographic regimes. Community structure, and particularly the pace of community dynamics, differ dramatically between intermittent upwelling regimes compared with relatively persistent downwelling regimes. We suggest that subsidy rates are a key determinant of the intensity of species interactions, and thus of structure in marine systems, and perhaps also nonmarine communities. Many ecological processes determine the structure and dynamics of communities and ecosystems. Theoretical and experimental advances have led to a growing awareness that different processes operate at different spatial and temporal scales (1, 2). Capturing the full richness of ecosystem dynamics thus requires studies ranging across a wide range of scales and allowing effective evaluation of the relative impact of the relevant factors. Doing so can be challenging. The logistics are daunting, especially in using the power of experimentation over large spatial scales. One solution, the ''comparative-experimental'' approach (3), involves replicated local-scale experimentation at multiple sites spanning larger scales, coupled with local-scale repeated sampling of factors that vary at characteristically larger scales.In its early development, marine community ecology focused on the dynamic consequences of primarily local-scale processes such as species interactions and physical disturbance (4). More recently, marine and nonmarine ecologists alike have documented the influence on communities of larger-scale phenomena including subsidies of materials and propagules transferred between adjacent ecosystems (5-8). Although evidence for the importance of subsidies is growing, questions remain about their impact, generality, magnitude, and interdependence and the physical and biotic mechanisms that underlie them.Here we use the comparative-experimental approach to address the role of large-scale oceanographic phenomena in structuring communities on New Zealand rocky shores. Based on earlier results (6, 9), we predicted that intertidal community structure and dynamics would reflect the coastal oceanographic regime. We hypothesized that the influences of oceanographically modulated subsidies (propagules, as a cause of increases in population density of benthic species, and the concentration of phytoplankton and detritus, as food for filter feeders) would be high with upwelling and low with downwelling. Study SystemPrevious research on the west and east coasts of the South Island of New Zealand (pairs of sites 100-500 m apart on each coast) revealed strik...

Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities

“…The supply of larvae establishes the densities of communities, while the supply of zooplankton determines the food supply for growth and reproduction of filter-feeding foundation species of rocky shore communities [3,35,36]. In turn, these subsidies determine the intensity of top -down processes in communities [4,37]. Hence, surfzone hydrodynamics as well as near-shore productivity may determine the initial abundance of communities that are later modified by top-down processes, physiological stress and disturbance [1].…”

“…Recently, alongshore variation in surfzone hydrodynamics was proposed to affect the recruitment of larvae to rocky intertidal populations in time and space, thereby affecting the intensity of postsettlement interactions among members of intertidal communities [7,8]. If so, spatial variation in surfzone hydrodynamics also may affect the concentration of entire plankton communities that provide food subsidies for suspension-feeding invertebrates, including foundation species that form habitat for many other species [3,4]. We previously demonstrated that the densities of barnacle and limpet recruits were greater onshore of dissipative than reflective surf zones [8].…”

Barnacle reproductive hotspots linked to nearshore ocean conditions

“…After decades of focusing on top-down (i.e., consumer-driven) effects on population and community dynamics, ecologists recognize that top-down and bottom-up (i.e., variation in nutrients and productivity) mechanisms act in concert to regulate ecological systems (18)(19)(20)(21). Nonetheless, empirical examples of bottom-up effects on marine populations and communities are relatively rare, in part because of the difficulty of manipulating potential factors on the appropriate spatial and temporal scales (3,20).…”

Latitudinal- and local-scale variations in a rocky intertidal interaction web

“…The strength and nature of biological interactions (such as competition, herbivory and predation) vary as a function of many factors (e.g. abiotic conditions, population densities and the genetic make-up of species; Thompson & Cunningham 2002, Menge et al 2003. These moderating processes of species interactions are themselves variable across large spatial scales, such as latitudinal gradients, and at more local scales (Crowe et al 2013, Doxford et al 2013.…”