IN SITU GROWTH RATES OF THE INFAUNAL POLYCHAETE POLYDORA CORNUTA IN THE TIJUANA ESTUARY

Includes bibliographical references (pages 55-65).Chapter 1. We developed methods to measure juvenile growth rates of the spionid polychaete Polydora cornuta in nature. We transplanted and recovered small vials containing labeled individuals and measured each worm's body volume before and after worms spent a week in field sediments. We transplanted labeled individuals to three elevations on an intertidal sandflat in Oneonta Slough within the Tijuana River National Estuarine Research Reserve: a high elevation 0.7 m above mean lower low water (MLLW), a mid elevation 0.4 m above MLLW, and a low elevation 0.1 m above MLLW. To include a range of environmental conditions, we performed two 7-d transplantations in July 2009 and two in November 2009. On average, we recovered one-third of the transplanted juveniles. Mean relative growth rates (RGR) declined significantly from 7.7 % d-1 at the low elevation to 2.4 % d-1at the high elevation. There was a positive correlation between RGR and the time each elevation was submerged, suggesting the decline in RGRs with increasing elevation was due to a reduction in the time available for suspension feeding. RGRs during one transplantation (12-19 November) were significantly slower than RGRs during the other three transplantations, especially at the high and mid elevations. Due to seasonality in the mixed semidiurnal tides, the 12-19 November period was the only transplantation that experienced lower low tides during the afternoon. The slower RGRs during this time of exposure to afternoon sunlight and temperatures suggest physiological stress may have contributed to reduced RGRs during this transplantation. Chapter 2. Sublethal predation can be a significant trophic pathway in sediment communities, and the regeneration of damaged tissue affects the activity, growth, and reproduction of prey. A field experiment was conducted 12-18 July 2010 in the Tijuana Estuary, California (+32.56617??, -117.13152??) to measure in-situ rates of body growth and palp regeneration of the polychaete Polydora cornuta after simulating sublethal predation by removing 2, 1, or 0 palps from labeled individuals. After 3 d in the field, individuals that had 2 palps removed grew significantly slower than worms that had 0 palps removed; 1-palp worms had intermediate growth. After 6 d in the field, rates of body growth were faster than those of worms recovered after 3 d, with similar trends among the three palp treatments. During the 3-d transplantation, removed palps regenerated to half the initial length of unmanipulated palps. After 6 d, palp lengths did not differ significantly among treatment groups, indicating complete regeneration. The regeneration rate of removed palps was significantly faster than the growth rate of undamaged palps, and palp-regeneration rate did not differ significantly between worms that had 1 or 2 palps removed. Chapter 3. Most spionid polychaetes are interface feeders that switch between suspension feeding in fast flows and deposit feeding in slow flows. Experiments in laboratory flumes show that growth rates of spionids increase with current speed and the flux of suspended food. These results have yet to be tested in the field where hydrodynamic conditions and other environmental variables are more complicated than in laboratory flumes. We constructed paired flow-manipulation channels (FMCs) that either increased or decreased the local velocities of the ambient tidal currents in Oneonta Slough, Tijuana Estuary. Paired FMCs were replicated in four trials that each lasted 4.3-9.0 days. During each trial, approximately 100 pre-measured Polydora cornuta were established in individually labeled vials that were transplanted into the subtidal sediment near the center of each FMC. At the end of each trial, recovered individuals were re-measured to calculate their relative growth rates. Acoustic dopler velocimitry (ADV) was used to measure 3-dimensional velocity and turbulence during ebb tide. On average, FMCs either increased ambient current velocity by 52% or decreased it by 45%. Calculations of turbulence (RMS) indicated that both types of FMC also altered turbulence relative to the ambient flow. In contrast to results from laboratory flumes, which were limited to velocities too slow to erode sediments, juvenile P. cornuta transplanted to FMCs that decreased the ambient velocity grew significantly faster than individuals transplanted to FMCs that increased the ambient velocity. We suspect the slower growth of P. cornuta in FMCs that increased ambient velocity was due to reduced particle capture during suspension feeding. Results of our field experiment suggest that P. cornuta grows fastest in intermediate flows, as shown for several bivalves and other suspension-feeding benthos