Inorganic nitrogen in the water column showed the following ranges: nitrate from 0.1 to 16.7 μM, nitrite from 0.07 to 0.6 μM, and ammonium from 0.16 to 1.7 μM (Fig. 2). Neither ammonium nor nitrite varied significantly with depth or temperature, whereas nitrate increased with depth, and showed an inverse relationship with temperature (Fig. 2a,b). Water between 15.8 and 20 °C never showed nitrate greater than 1 μM, whereas water that was colder than 15.8 °C did. Therefore for this study we considered water below 15.8 °C to contain detectable nitrate for our conservative temperature proxy.