Runoff from tomato cultivation in the estuarine environment: biological effects of farm management practices

Abstract

The use of plastic row covers (plastic mulch) on vegetable farms increases runoff of pesticides after rainfall events and has been linked to toxic events in adjacent tidal waters. In coastal Virginia, USA, runoff from tomato fields with plastic mulch was suspected of causing mortality of commercial hard clam larvae at a hatchery located downstream of farming operations. Concern about the putative impacts of this practice on local waters resulted in a collection of studies to: (1) determine the sensitivity of early life stages of bivalves to copper, a commonly used fungicide; (2) examine acute and chronic biological effects of runoff on tidal creeks; and (3) examine the efficacy of management practices designed to reduce the delivery of pesticides to adjacent creeks. Laboratory bioassays revealed that 48-h LC 50 values for embryonic clams Mulinia lateralis and Mercenaria mercenaria were 38 and 20 μg/l, respectively. In situ bioassays with Palaemonetes pugio showed that pulsed toxic conditions sometimes occur downstream of some tomato farms in plastic mulch following rainfall events. Growth, mortality rates and bioaccumulation of copper and organic pesticides in oysters were not correlated with the use of plastic mulch in watersheds. Sediment bioassays indicated potential toxicity in sediment collected downstream of some tomato fields in plastic mulch, but the effects were not consistent between years. Closer examination of management practices on the farms suggests that controlling runoff can prevent toxic impacts. Elevated levels of crop protectants measured at the outflow of farm ponds suggested that they may do little to reduce loadings of some pesticides. However, forested buffer zones and ephemeral sedimentation basins appeared to be effective in reducing pesticide concentrations in runoff and pulsed toxicity in tidal creeks.