Behavioural response to gradually declining oxygen concentration by Baltic Sea macrobenthic crustaceans

Behavioural responses to decreasing oxygen concentrations were studied in the amphipods Monoporeia affinis (Lindström) and Pontoporeia femorata Krøyer and the isopod Saduria entomon (L.), all benthic macrofauna species from the Baltic Sea. Infrared time-lapse video recording was used. Seven oxygen concentrations were studied, starting near saturation (about 90%), followed by about 65, 49, 33, 16, 8 and 5% oxygen saturation, with each oxygen level maintained for 24?h. M. affinis was more active than P. femorata. The duration of swimming activity of M. affinis was shorter at ≤33% oxygen saturation compared with at the two highest levels. The duration of swimming by P. femorata was longest at 90, 65 and 5% oxygen saturation. For both amphipods the frequency of swimming activity was highest at 5% oxygen saturation. Above 7% oxygen saturation, S. entomon spent most of its time in the sediment, whereas at levels ≤7% it was most often found on the sediment surface. Both amphipods were predominantly night active (nocturnal), whereas no diurnal variation in behaviour was detected for S. entomon. It is suggested that the amphipods minimise the costs associated with obtaining oxygen as well as their risk of predation by moving little on or above the sediment until a lower critical oxygen level is reached. Below this level, they show an oxygen-seeking behaviour. S. entomon also hid in the sediment for as long as possible, suddenly coming to the surface once the oxygen concentration became intolerably low. Its behaviour is also indicative of a trade-off between predation risk and obtaining sufficient oxygen. It is suggested that the change in behaviour shown at about 33% oxygen saturation could lead to a long-term alteration in community structure in the deeper parts of the northern Baltic proper, whereas the change at about 8% oxygen saturation reflects a more acute, short-term response.