Decomposing medusae as substrata for bacterial growth and their potential contribution to environmental hypoxia

Abstract

Decomposing jellyfishes at 9ᵒC showed exponential loss in wet and dry weight, particulate (POC) and dissolved (DOC) organic carbon and particulate organic nitrogen (PON), with turnover times of 3.5–4.2 d. The amount (%) carbon and nitrogen of carcasses, decreased linearly over time, but the C/N atomic ratio remained stable at 7. The daily decline of POC and PON, as a % of starting C and N, both showed a negative relationship with medusa weight, but with a stable POC/PON atomic ratio at 9.1. Five morphological groups of bacteria that co-occurred with the medusa, all exhibited exponential growth, with an average daily biomass doubling rate between 2.8 and 4.9. Over a 3-d period the biomass ratio between coccoids and spirills increased by a factor of 76. Oxygen consumption in the surrounding water was on average 0.34 ± 0.10 mg l−1 h−1 and slightly increasing over time. Assuming the oxygen consumption being a function of bacterial respiration, the bacterial growth efficiency increased exponentially from <3% during the first 25 h to 16% after 55 h. Incorporating all of these results, the potential impact of decomposing jellyfishes on environmental oxygen concentration could be simulated, with the biomass of decomposing medusae and water-mixing volume as input variables. Such simulations would be useful to forecast environmental hypoxia.