ECOLOGY OF THE MARINE CLADOCERAN PENILIA AVIROSTRIS (dacha.atienza@gmail.com)
The main objective of this Thesis was to get new insights into the ecology of Penilia avirostris in order to characterize and better understand the ecological role of this marine cladoceran on the planktonic food web, the energy flow, and biogeochemical cycles. The hypothesis behind is that in spite of the sporadic and opportunistic appearance of P. avirostris populations, its occurrence will have an important impact on the circulation of organic carbon on the planktonic marine food webs. This Thesis covers a wide view of P. avirostris including topics on distribution, seasonal pattern, grazing, metabolism, life cycles, and impact on the plankton communities in the water column. P. avirostris showed a clear seasonal patter in the NW Mediterranean, being an important component of the mesozooplankton during the stratification period. Specific horizontal and vertical patterns were identified. Higher abundances of P. avirostris appeared in the southern region due to the presence of low salinity waters, high chlorophyll concentrations, and higher temperatures. Lower abundances occurred on high salinity waters (>38, oceanic waters) or with low temperatures (<22 ºC). P. avirostris appeared in the upper 60 m of the water column, being more abundant between 20 and 30 m. Life cycle of this marine cladoceran is characterized by two types of reproduction, parthenogenesis and gametogenesis. Parthenogenesis seems to be the responsible for the explosive growth of P. avirostris populations when conditions are favourable. Brood size varied between 1 and 8 embryos female-1, and developmental time was estimated around 2 days. Both parameters rendered in higher birth and population growth rates. Shift between parthenogenetic reproduction to gametogenesis was associated to the disappearance of P. avirostris from the water column. The results indicate that P. avirostris feeds on particles on a wide size range, mostly on nanoplankton (2-20 µm), including larger prey such as dinoflagellates and ciliates. Autotrophic and heterotrophic bacteria were not grazed on, but picoflagellates (<2 m) were significant contributors to diet, indicating a narrow lower prey size threshold. Although this marine cladoceran is considered to behave like a filter feeder, our results indicate that it can display variable selectivity patterns not depending only on prey size. During oligotrophic conditions daily rations of Penilia avirostris ranged between 26% and 157% body carbon d-1. Trophic impact of P. avirostris accounted, on average, for <10% of the standing stock of each of the microbial groups considered. The results indicate that P.avirostris does not excrete phosphorus, but it does ammonia, with the consequent higher recycling of N when it dominates the water column. Finally, gross growth efficiencies (GGE) varied between 16 and 58% which is not different to other zooplankters, such as copepods, and the ecological success of this species is related more with the life cycle and feeding performance than with GGE.