The measured concentration in sampled pore waters is likely to significantly overestimate the exposure to ammonia by epibenthic and benthic organisms inhabiting surface sediments (0–10 mm),while organisms burrowing deeper irrigate their burrows with
oxygenated overlying water, so also have a reduced exposure to ammonia. Furthermore, determining a suitable chronic effects threshold for benthic organisms is not possible using the available effects data because, firstly, the ammonia sensitive early life stages of a large proportion of benthic organisms are pelagic (in the water column) and, secondly, many benthic organisms receive the majority of their exposure from the overlying water (via filtration),rather than directly from sediment pore waters.