Measuring in situ growth rates of an infaunal polychaete at different intertidal elevations

Abstract

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 3 elevations on an intertidal mudflat 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 2 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 1 transplantation (12 to 19 November) were significantly slower than RGRs during the other 3 transplantations, especially at the high and mid elevations. Due to seasonality in the mixed semidiurnal tides, the 12 to 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 air temperatures suggest physiological stress might have contributed to reduced RGRs during this transplantation. © Inter-Research 2011.