PROCESS-SPECIFIC CUES FOR RECRUITMENT IN SEDIMENTARY ENVIRONMENTS - GEOCHEMICAL SIGNALS

The most biologically and geochemically active marine sediments are ch aracterized by steep chemical gradients within the top centimeters of sediment (Berner, 1980). A common feature of these environments is dis ruptions of surface sediments by both physical and biotic forces. Grow th and mortality rates for new recruits are affected by many of these surface perturbations. At the same time, these disturbances also impos e a discontinuity in concentration across the sediment-water interface , and accordingly, a change in surface chemistry. In this paper we pre sent evidence that the cue used by juveniles to distinguish between re cently disturbed and undisturbed surfaces may be disruption of geochem ical gradients that are typical of nearshore benthic systems. New juve niles exposed to ammonium concentrations typical of disturbed surface sediments exhibit behaviors consistent with rejection of the habitat. Conversely, new juveniles placed onto sediments containing ammonium le vels typical of undisturbed surficial sediments rapidly initiate burro wing activity, a sign of ''acceptability.'' We also present a numerica l model, which assesses the dynamics of small-scale chemical shifts th at accompany sediment disruption, to determine (a) what is the magnitu de of surface chemistry changes associated with disturbance (i.e. what is the signal strength)? and (b) what are the spatial and temporal sc ales associated with the return to the undisturbed condition (''recove ry'')? Model results show that the signal strength, and the return to ''acceptable'' conditions, are strongly influenced by the initial grad ient. Model predictions of the time required to ''recover'' indicate t hat times to recovery are longer than the interval between disturbance events, but are of the same temporal scale (minutes to hours). Thus, our results suggest that the dynamics of surficial gradients provide a strong signal over appropriate time scales that may reveal the intens ity of disturbance and the likelihood of mortality for juveniles. As s uch, transport-reaction processes which govern porewater concentration s in surficial sediments may also play a role in recruitment processes .