Mechanisms of Bio-Physical Coupling at Submarine Bank Ecosystems. ICES CM 2007/B:10
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Not to be cited without prior reference to the authors ICES CM 2007/B:10 Mechanisms of bio-physical coupling at submarine bank ecosystems Christian Mohn and Martin White Dept. Earth and Ocean Sciences, National University of Ireland, Galway, Ireland Abstract Submarine banks, like many other isolated or quasi-isolated topographic features have long been recognized as important hot spots of bio-physical interactions. At a smaller spatial and temporal scale, physical processes can have a significant effect for chlorophyll and benthic dynamics. A sufficiently long residence time of primary production is important for any sessile benthic community resident over the topography to (i) transfer surface productivity to higher trophic levels or to (ii) transfer organic material directly to benthic communities before it is lost to the system. An important benthic ecosystem at the Rockall and Porcupine Bank slopes at the European continental margin is that of cold water corals. These reef-forming corals are generally found in regions of strong benthic currents and enhanced surface productivity. We analysed several years of remote sensing data (SST, Chlorophyll-a) to identify robust bio-physical distribution patterns. In a second step, data from recent surveys (ADCP and current meter mooring data) and results from model simulations were used to investigate the relative importance of physical processes on various spatial and temporal scales (rectified flow, tides, internal waves) as a possible feeding mechanism for benthic communities at these locations. 1. Introduction The oceanic regions west of Ireland have always been considered as an important area for the accumulation and propagation of warm and saline North Atlantic waters to the Arctic Ocean where they are transformed to cold, fresh deep water and contribute to the North Atlantic thermohaline overturning circulation. They are also a transition zone between the subpolar and subtropical gyre systems. A wide spectrum of localised dynamics associated with the complex and abrupt topography of the region is superimposed to the far field forcing, including strong barotropic and internal tidal activity, up- and downwelling events and eddy activity. These processes act together to support a variety of biological phenomena and patterns, such as enhanced primary and secondary production as well as aggregation and retention of biological material and higher tropic organisms. In this study, a special focus is given to processes and patterns associated with the large submarine banks including the Porcupine Bank / Irish shelf transition zone and the Rockall Bank (see Fig. 1). We introduce and summarize concepts and mechanisms of physical forcing as an important contributor for biological variability to illustrate the importance of a better understanding of the different aspects of bio-physical coupling in the region. o 60 N RB 0.2 57oN 0.5 1.0 RT 2.0 o 54 N 3.0 0.2 IS 4.0 1.0 0.5 o 51 N PB 2.0 0.1 PS CS GS 48oN 45oN 20oW 18oW 16oW 14oW 12oW 10oW 8oW 6oW 4oW Figure 1: Map showing the study area. Depth contours are in km and topographic features are labelled CS (Celtic Shelf), GS (Goban Spur), IS (Irish Shelf), PB (Porcupine Bank), PS (Porcupine Seabight), RB (Rockall Bank) and RT (Rockall Trough). 2. Material and methods Based on a combination of literature review and data analysis we describe the spectrum of physical processes associated with the submarine banks of the Rockall Trough and their potential to affect biological material transport as well as particle aggregation, trapping and retention. We analysed 8 years (1998-2005) monthly composites of remotely sensed SST and Chlorophyll-a (SeaWiFS) data to identify spatial bio-physical distribution patterns and their temporal variability. These data were combined with results from model simulations using the 3-dimensional ocean circulation model SPEM (s-coordinate primitive equation model). To describe the different aspects of benthic dynamics we reviewed results from early and recent studies and summarized possible implications for benthic ecosystems. 3. Physical controls The most notable circulation feature at the continental margin is the shelf edge current (SEC), defined by a distinctive high salinity core centered at depths between 200 - 500 m (e.g. Hill and Mitchelson-Jacob, 1993). The inter-annual signal is the dominant mode of the SEC variability. In the Rockall Channel it is described as a narrow (20-50 km), but steady poleward boundary current with typical velocities varying between 10 cm/s in summer and up to 30 cm/s in winter (e.g. Booth and Ellett, 1983). The SEC is less energetic at the more southerly regions between the northern Bay of Biscay and Goban Spur (Celtic and Armorican slopes) with some evidence of occasional reversal of flow: Pingree and Le Cann (1990) identified a persistent northwestward slope residual current of 6 cm/s accompanied by a weak southeastward counter-current of 2 cm/s on the outer Celtic shelf. At Goban Spur, maximum poleward flow also occurs in winter (December – January), whereas a remarkable modulation of the upper slope current amplitude and direction is manifested in spring (March-April) and autumn (September-October). In these periods the northward along-slope flow turns into an equatorward flow with occasional inertial overshooting into the deep oceanic regions west and north of Goban Spur (Pingree et al., 1999). This seasonal variability pattern is known as SOMA (September-October, March-April) and occurs as a response to seasonal changes of the local mean wind stress field. However, this response mainly affects the upper SEC layers, whereas near-bottom flow appears to be more topographically controlled with a generally northwestward and along-slope direction (Pingree et al., 1999). The Porcupine Bank and Porcupine Seabight region west of Ireland is generally considered as a critical area for the continuity of the SEC. A continuous poleward flow along the Porcupine Seabight margins is less readily observed. However, there is observational evidence for a weak mean along-slope flow in the near-bottom layers of the eastern and northern Porcupine Seabight in the order of 5 cm/s. West of Porcupine Bank, the SEC is apparent at all available depth levels with an average velocity of 10 cm/s. At the northern Porcupine Bank poleward flow again increase to magnitudes which are frequently observed along the Scottish slope (e.g. White and Bowyer, 1997; Mohn, 2000). Porcupine and Rockall Bank are large submarine topographic features at the entrance of the Rockall Trough. Whereas Rockall Bank is largely isolated from its surroundings, Porcupine Bank is attached to the shelf break, separated from the Irish continental margin by a shallow (300 m deep) channel (see Fig. 1). Systematic observations over longer periods are sparse, but scientific attention to these areas largely improved over the last decade with the discovery of giant carbonate mound provinces and reef-forming cold water corals with species-rich benthic communities (Roberts et al., 2006). A frequently observed feature at both banks is a region of cool temperatures above the summit regions compared to warmer waters of the surrounding oceanic areas (e.g. Mohn and White, 2007). The cold cores are associated with a closed, clockwise, bottom-intensified recirculation cell along the bank slopes (see Fig. 2). The time-mean solution of this phenomenon is commonly referred to as a Taylor cap and is generated by impinging far field currents and/or the resonant generation of large amplitude topographically-trapped waves through diurnal tidal currents. It has been described for tidally dominated regimes (e.g. Beckmann and Haidvogel, 1997) as well as locations with prevailing low-frequency forcing (e.g. Chapman and Haidvogel, 1992). Enhanced turbulent vertical mixing (e.g. Kunze and Toole, 1997), internal tidal activity and uplifting of cold, nutrient rich water (e.g. Genin and Boehlert, 1985) also contribute to the local flow dynamics and may generate favourable conditions for enhanced biological activity and biodiversity (e.g. Genin, 2004). At Porcupine Bank, the clockwise recirculation pattern is most likely generated by a combination of diurnal tidal rectification and the influence of the SEC. However, it is strongly modulated by seasonal changes of the far field forcing. In summer, tidal rectification is the major contributor to the observed flow pattern over the bank, whereas in winter, with the intensification of the SEC, tidal influence is decreasing and a more asymmetric recirculation with enhanced flow along the western slopes can be expected. In addition, the Taylor cap flow over Porcupine Bank is weak in periods of strong stratification and strengthens towards winter when the seasonal thermocline is replaced by vertically mixed surface layer (Mohn et al., 2002). White et al. (1998) found that a Taylor cap and associated doming of cold, nutrient rich water over Porcupine Bank persisted from April to July 1995. However, Kloppmann et al. (2001) demonstrated that severe storms can partially destroy the Taylor cap over the bank summit with significant consequences for the biological environment. A steady impinging flow of similar magnitude as the SEC is not present at Rockall Bank, and tidal rectification is assumed to be the dominant process for the formation of a time-mean along-isobath flow (Mohn and White, 2007). A comprehensive image of the mean flow at Rockall Bank based on observations alone is not available for many parts of the bank. However, from a series of historical (Dooley, 1984) and recent current meter data (White, pers. comm.) there is strong evidence for a symmetric, clockwise residual re-circulation centered at the 500 m isobath with maximum speeds of up to 20 cm/s. In addition, strong amplification of the major diurnal tidal constituent K1 seems to be a persistent phenomenon at or near the topographic boundaries of Rockall Bank. Historic observations by Huthnance (1974) revealed a up to 20-fold amplification of the K1 tide compared to typical amplitudes in the oceanic far field outside the bank, recently supported by measurements of White (pers.