Mortality and bacterial decomposition of pelagic copepods associated to Oxygen Minimum Zones (OMZ)
Fecha
2018Autor
Hidalgo-Diaz, Pamela del Carmen
UNIVERSIDAD DE CONCEPCION
Institución
Resumen
Mortality is perhaps at least constrained in copepod population dynamics. Traditional research for convenience assumes that mortality is driven solely by predation and therefore can be derived from changes in population abundances. A corollary to this practice is that field sampling simply ignores the live/dead status of the animals. It is, however, illogical to believe all copepods in situ are alive. Copepods and other zooplankton can suffer non-predation mortality that leaves behind carcasses. A meta-analysis of literature data suggests that up to one-third of in situ copepod mortality cannot be explained by predation. Ignorance of carcasses occurrence also causes errors to other population parameters because dead copepods obviously do not molt, grow or reproduce. A modeling study showed that ignoring even a small magnitude of carcasses abundance and non-predation mortality could lead to unrealistic projection of population growth. Recent advances in staining methods for distinguishing between live and dead individuals in field samples opened the opportunities to make detailed quantification of copepod carcasses in the Humbolt Current System (HCS). A high percentage of zooplankton mortality associated with the Oxygen Minimum Zone (OMZ) present in the HCS, could increase the abundance of copepod carcasses, representing a concentrated pool of labile organic substrates for bacterial hotspots. Deceased copepods tend to sink because of higher density than surrounding water, but sinking is modulated by bacterial decomposition. Bacterial decomposition remineralizes carcasses’ organic materials and could support high bacterial production from anaerobic processes into the OMZ, which do not occur in the oxygenated water of surface layers. This can serve as a hint for evaluating alternative pathways of material and energy fluxes in the classic food web into the OMZ, presenting copepods and bacteria as directly connected functional groups. The present research, from an experimental perspective, dealt with copepod and bacterial composition of the calanoid copepod Acartia tonsa under oxic and anoxic conditions. From the in situ perspective, the non-predatory mortality of dominant pelagic copepods associated with the OMZ was studied in northern Chile within the HCS. The results found were the following: (1) Using detailed data of live/dead compositions, along with stage durations and molting rates, we derived both predatory and non-predatory mortality rates of the three main copepod species, Paracalanus cf. indicus, Acartia tonsa, and Calanus chilensis, within the Chilean Humboldt Current System (HCS), and examined their relations with the environmental factors for the first time. Total mortality rates, calculated as the sum of average predatory and non-predatory mortality rates over the two annual cycles, increased with developmental stages in all three species. The total mortality rates of C5 were 3-4 times higher than those of C1. On average, predation accounted for 53.7 of the total mortality in P. cf. indicus, 56.4% in A. tonsa, and up to 65.2% in C. chilensis. Meantime, non-predatory mortality accounts for 34.8 to 46.3 % of the total mortality, and this reflects the importance of environmental stresses in controlling the copepod population dynamics within the HCS; (2) From laboratory experiments, we followed the course of decomposition of the carcass of the marine copepod A. tonsa. In oxic conditions, decomposition began rapidly, later it remained relatively stable, and slowly increased towards the last hours of the incubations. In contrast, the decomposition process was much slower in the anoxic conditions, increasing greatly and quickly over the last hours of the incubations. Different bacterial communities among different treatments of decomposition experiments were found, whereas changes and succession were observed throughout both incubations. Using Illumina sequencing of the 16S rRNA gene we identified members of the bacterial community associated with carcass decomposition of A. tonsa. From the phylogenetic analysis, it could be concluded that the investigated copepod was associated with bacteria of three phyla: Proteobacteria, Bacteroidetes, and Verrucomicrobia. Vibrionaceae, Pseudoalteromonadaceae, Rhodobacteraceae, Flabobacteriaceae and Verrucomicrobiaceae were the most abundant bacterial family. Expression of nirK and nosZ marker genes for denitrification (N2 and N2O production, respectively) were found to occur under anoxic conditions and showed an increase with time, mainly associated with the late stage of decomposition. Then, the investigation of bacteria associated to the decomposition of copepod carcasses is very important, due to the functioning of those carcasses as pelagic hotspots for bacterial decomposition may have implications in nitrogen cycling via denitrification; and (3) Another laboratory study showed that the production of NH4+ from the decomposition of carcasses was observed in all treatments under low oxygen saturation levels and came from mineralization processes; thus, DNRA may contribute. Moreover, the production of NH4+ further suggests that sinking of copepods’ carcasses passively transports bound nitrogen to depths where it could be partially released as ammonium and may fuel free-living anammox bacteria and thus contribute to pelagic N-loss by anammox. Then, pelagic N-loss is potentially enhanced by carcasses of chitinous zooplankton directly through N2 and N2O production, as well as indirectly through NH4+ production that may contribute to anammox. As copepods are some of the most abundant mesozooplankton in the oceans, anaerobic nitrogen cycling associated with these animals may have large implications for biogeochemical cycling in the oceans, and the fixed-nitrogen loss from marine pelagic environments.