Ammonium excretion rates of giant California sea cucumbers (Apostichopus californicus)

Abstract

Nitrogen is a key nutrient used to support the growth of primary producers. In the ocean, it can be supplied in various ways, including through decay, upwelling, nitrogen fixation, and via animal excretion. There is a possibility that ambient nitrogen availability in nutrient-rich cold waters could be enhanced by animal-mediated nitrogen production. Here, we used giant California sea cucumbers, Apostichopus californicus, to explore animal-mediated nitrogen contribution in temperate oceans by holothurians, asking specifically if (a) nitrogen levels decrease with increasing distance from sea cucumbers, and (b) larger sea cucumbers excrete ammonia at higher rates. We analyzed ammonium concentrations using fluorometry water samples collected internally, near (3-5 cm) and far (1 m) from sea cucumbers at Reed Point Marina, with average ammonium concentrations of 2.79 µmol (SD = 0.75 µmol) and 3.21 µmol (SD = 1.28 µmol) respectively. There was no significant difference in ammonium concentrations near and far from sea cucumbers. Ammonium concentrations in the two internal samples obtained were ~ 7 times higher (22.96 ± 2.12 µmol) than the ambient samples, suggesting a strong dilution of sea cucumber nitrogen upon excretion. There was a strong positive correlation between excretion rate and sea cucumber mass (R² = 0.79). Based on our findings we are still unsure of the ecological impacts of sea cucumber fishing, which is becoming more prevalent in the northeast Pacific, as we can only see that giant California sea cucumbers at an individual scale do not contribute a significant amount of nitrogen. Further research needs to be conducted as the role of a species in its environment is key to learning the potential ecological repercussions of removing it.