Pulsed phytoplankton supply to the rocky subtidal zone: influence of internal waves.

Abstract

Hydrographic measurements indicate that the thermocline and the phytoplankton-rich chlorophyll maximum layer are vertically displaced over a rocky pinnacle in the central Gulf of Maine by internal waves with maximum amplitudes of 27 m. Such predictable downwelling events are linked to rapid, 2-to 3-fold increases in chlorophyll a, an indicator of phytoplankton concentration, in pulses of warm water recorded 4 cm above the bottom (29-m depth). The 1.5-5.6'C temperature fluctuations had an average period of 10.6 min and were generated on both ebb and flood tides. Local lee waves and the arrival of solitons propagated from Georges Bank are hypothesized to explain the timing of the internal waves. Because internal waves and chlorophyll mxima are pervasive features of stratified temperate seas, this mechanism of food coupling should be common in other rocky subtidal habitats.Despite the long-acknowledged dependence of bottomdwelling populations on pelagic processes (1), relatively little is known about how events in the water column influence organisms inhabiting rocky subtidal habitats along the underwater coastline of continents at high latitudes and on offshore ledges, banks, and seamounts. Recent benthicpelagic coupling research has linked the impingement of an oxygen minimum layer (2) and topographically enhanced currents (3) to the vertical zonation of invertebrate communities on Pacific seamounts. The importance of understanding the process of food supply to the sea floor is underscored by the finding that the input of particulate organic carbon can regulate the size and distribution of populations in soft bottom habitats (4, 5). Moreover, the timing ofphytoplankton supply is thought to influence the evolution of benthic invertebrate reproductive strategies (6).Internal waves are generated when the ebb tide forces a shallow thermocline over the edge of a steep bottom feature such as a bank, ledge, or continental shelf (7). A prominent depression ofthe thermocline is produced on the downstream side and held over the abrupt bottom until the ebbing currents slacken. The released depression propagates away from the topographic feature as a packet of internal waves (8, 9). Because of their ability to move phytoplankton-rich chlorophyll maximum layers downward (10-12), internal waves are a likely mechanism of benthic-pelagic food coupling. To date, investigations of internal wave effects on phytoplankton distribution have been restricted to the water column, and it is not evident that downwelled chlorophyll maximum layers could reach the rough surface of rocky bottoms where topographically induced upwelling could redirect downward flow (13-15).We studied benthic-pelagic coupling at Ammen Rock Pinnacle (ARP), a rocky ledge located 105 km offshore in the central Gulf of Maine characterized by steep bottom topography, swift currents (averages, 12.7-25.5 cm/sec) and abundant populations of suspension-feeding sponges, anemones, bryozoans, and ascidians ( Fig. 1 and refs. 16 and 17). Observations of rapid vertical...

Depth-specific differences in growth of the reef sponge Callyspongia vaginalis: role of bottom-up effects

“…Our study suggests that bottom-up processes may be important to sponge bioenergetics and energy transfer on Conch Reef, but more work is needed to identify the biological and/or physical processes responsible for increased food availability at deeper depths. Other studies have indicated that internal wave activity on Conch Reef (Leichter et al 1996(Leichter et al , 1998(Leichter et al , 2003 and elsewhere (Witman et al 1993(Witman et al , 2004) may enhance food availability at deeper depths during the stratified season. However, because our measurements were performed during non-internal wave conditions, this phenomenon probably plays an ancillary role in producing depth-specific differences in food availability.…”

Breaking internal waves on a Florida (USA) coral reef:a plankton pump at work?

“…These phenomena are widespread in both coastal and offshore oceans (Wolanski 1994, Pineda 1995 and represent an important mechanism of high frequency mixing and transport across density discontinuities (Sandstrom & Elliot 1984, Wolanski 1994, Shanks 1995. In the rocky subtidal of the Gulf of Maine, unbroken internal waves can produce downward, vertical deflection of the thermocline that brings a subsurface chlorophyll maximum layer into contact with dense aggregations of benthic suspension feeders (Witman et al 1993). While floating, neustonic material may be transported in surface slicks produced by trains of subsurface internal waves (Shanks 1983, 1986, Kngsford & Choat 1986; horizontal transport at the pycnocline by internal waves in deep water should be slow at best.…”

Pulsed delivery of subthermocline water to Conch Reef (Florida Keys) by internal tidal bores

“…Frederiksen et al (1992) suggested suspended particle delivery by internal waves could explain patchy distributions of a deep-water scleractinian coral in the northeastern Atlantic. Witman et al (1993) documented transport of warm, phytoplankton-rich water to dense aggregations of sessile suspension feeders on shallow pinnacles in the Gulf of Maine. Andrews and Gentien (1982), Wolanski and Pickard (1983), and Wolanski (1994) have pointed to tidal oscillations of the thermocline in the Coral Sea as a potential source of nutrients for the outer shelf of the Great Barrier Reef, and Novozhilov et al (1992) reported evidence of tidal upwelling near reefs in the Seychelles Islands.…”