Description
Southern California has few major rain events each year, and those that do occur generate stormwater runoff that impacts coastal environments in many ways. To measure seasonal variation of in situ growth rates we measured juvenile growth rates of the spionid polychaete Polydora cornuta in situ by transplanting and recovering small vials containing labeled individuals and measuring each worm's body volume before and after week-long transplantations into field sediments. We performed 15 intertidal transplantations between November 2008 and January 2010 in the Oneonta Slough within the Tijuana River National Estuarine Research Reserve. Our study period included four major rain events with precipitation greater than 2 cm d__ that reduced salinity in the tidal channel below 20%. Mean relative growth rates ranged from 0% to 15% d__ among the 15 transplantations and were positively correlated with the mean salinity during each transplantation period. Mean relative growth rates during or a few days after major rain events averaged 2% d__, while relative growth rates during all other weeks exceeded 7% d__. It is suspected that the reduced RGR of worms during periods of low salinity results from a cessation of feeding activity and worms withdrawing into their protective tubes below the sediment-water interface. Additionally, we measured the concentrations of 13 metals in sediment and tissues of P. cornuta in Oneonta Slough purposefully before and after major rain events throughout the 2008-2009 rainy season in order to assess seasonal variations. Surprisingly, concentrations of metals in samples of sediments and polychaetes did not vary significantly in relation to these major rain events. Presumably, stormwater runoff at this site is ephemeral enough that it does not alter the background concentrations of metals in the sediment significantly. Evidence from the previous study concluded that worms did not feed during these periods of very low salinity. This would reduce the mechanism by which stormwater runoff would be expected to alter the short-term concentrations of metals in the tissues of these worms. To test the effects of water flow speed and aqueous copper on the feeding behavior, growth rate, and accumulation of copper in the tissues of P. cornuta we performed an experiment in a laboratory flume. The experiment included two flow speeds (6 or 15 cm/s) and two concentrations of added copper (0 or 85 _g/L). Worms grew significantly faster in the faster flow and in the lower copper concentration. In the slower flow, the total time worms spent feeding decreased significantly as copper concentration increased, but copper did not significantly affect the time worms spent feeding in the faster flow. Across all treatments, there was a significant, positive relationship between the time individuals spent feeding and their RGR. Worms were observed suspension feeding significantly more often in the faster flow and deposit feeding significantly more often in the slower flow, but copper concentration did not affect the proportion of time spent in either feeding mode. The addition of 85 _g/L copper significantly increased copper accumulation in P. cornuta tissue, but the accumulation did not differ significantly due to flow speed. There was, however, a significant interaction between copper and flow; the magnitude of the difference in copper accumulation between the 0 and 85 _g/L treatments was greater in the faster flow than in the slower flow. In slow flows that favor deposit feeding, worms grow slowly and accumulate less copper in their tissue than they do in faster flows that favor suspension feeding and faster growth.
