INTERNAL TIDAL BORES IN THE NEARSHORE - WARM-WATER FRONTS, SEAWARD GRAVITY CURRENTS AND THE ONSHORE TRANSPORT OF NEUSTONIC LARVAE

Nearshore temperature fluctuations are associated with energetic cross-shore two-way flows that influence the onshore transport of neustonic larvae. Water temperature near the surface and bottom at two nearshore stations off southern California (6 and 15 m water depth, respectively) can drop sharply and subsequently rise. Two or more consecutive drops and rises can occur at diurnal or semidiurnal periodicities. The temperature increases may be accompanied by energetic seaward bottom currents together with sharp-edged warm-water fronts. (Warm-water fronts are defined here as linear seasurface features dividing parcels of water of different temperature.) Shoreward-moving surface fronts divided bodies of water of different surface temperature, where the coldest water body was inshore. Fronts disappeared at (or close to) the surf zone. The sharp drops in water temperature are interpreted as the onshore advection of subsurface water by large internal tidal bores, and it is concluded that the sudden increases in temperature and cross-shore advection are epiphenomena of internal tidal bores. Internal tidal bores have been invoked previously to explain the onshore transport of water-column larvae. This study tests the hypothesis that shoreward surface flow, an epiphenomenon of internal tidal bores, transports neustonic larvae in warm-water fronts. Five warm-water fronts were sampled in shallow water (about 6 m) for temperature and fish and crab larvae in June-July 1992. These larvae were more abundant in fronts than in parcels of water preceding or following the front. Peaks in larval abundance were accompanied by a sharp rise in temperature, in itself evidence for onshore transport of surface water. It is concluded that both warm-water fronts and internal tidal bores play a key role in the exchange across the shelf of material and water properties, and that internal tidal-bore phenomena explain well the transport of both water-column and neustonic larvae in different habitats.