Sustainability Issue #2 July 2009

On the basis of data which has been so far analysed, it appears that two species which are new to this area have had a very great impact on the ecosystem. Both are warm water species and arrived here over the past seven years. The cladoceran Penilia avirostris was detected in the fjord for the first time in 2002, and since then is has become common in other Scandinavian waters. It has been well researched in other regions and specialises in eating very small plankton and to some extent even bacteria. It is therefore radically different from the common water fleas Podon sp  and Evadne sp. Penilia has a population plateu in the autumn and then dominates over the otherwise common copepods. It is difficult to say what effect they have on the ecosystem, but their food choice indicates that they can function as a link between the microbial loop (bacteria-picoplankton-nanoflagellates) and higher levels in the food chain. Their behaviour is also more passive than that of copepods. They appear to be not as good at escaping from attacks. This is perhaps an advantage for zooplankton feeders which can catch their prey more easily.

Comb jellyfish

The other important species is the comb jellyfish Mnemiopsis leidyi which has been widely discussed and maybe also has a greater impact. There are fears that it will wipe out other zooplankton and thus reduce the amount of food for fish larvae and planktivorous fish. In Gullmarsfjorden the comb jellyfish was found for the first time in the autumn of 2006 and has since then steadily increased in numbers. It appears able to overwinter in the fjord and has its population plateu in August-October. As larvae, Mnemiopsis mostly eats phytoplankton and the smallest zooplankton (microzooplankton, 20-200 mm). When they are over 1 cm, their food mostly consists of copepods. In the autumn of 2008 Mnemiopsis was very common in surface waters and it could then reduce the number of copepods to between 1 and 10% of the normal levels. What is interesting is that this comb jellyfish appears to be selective in its predation and it is the largest and fastest swimming species of the copepods (Centropages typicus) that is most exposed.

Mnemiopsis impacts on several levels of the food chain in Gullmarsfjorden. When copepods practically vanished in September, the grazing pressure on their food, phytoplankton, also disappeared. The phytoplankton could therefore greatly increase its biomass, although the actual growth of algae did not increase. Algal blooms of dinoflagellates are common in the autumn, but were not observed in 2008, and diatoms instead dominated. Chlorophyll contents were  also higher than normal. This suggests that Mnemiopsis can act as a predator at the top of the planktonic ecosystem on the west coast, and when it is common in the autumns, its impact can propagate right through the food web. Such "trophic cascades" are manifested by the suppression of their prey in the biomass while at the same time the next lower trophic level is released from predation and increases. This results in negative correlations between the biomass of adjacent trophic levels, every other level being favoured and every other disfavoured. We observed this in the autumn of 2008. This is fundamentally different from nutrient supply effects which propagate right through the food web and benefit all levels.

Two new species

Important questions now are what the annual cycle of Mnemiopsis is like. It is evident that it is common in August, but where is it during the rest of the summer? One strategy may be that it is present in deeper regions until some factor alters its behaviour and it comes up to the surface. Another explanation may be influx with currents from open sea regions and that this occurs in August. With two "new" species in the free water mass of Gullmarsfjorden, much can happen. We are now focusing our research on the role of Penilia in the community and on the interaction between Penilia and Mnemiopsis. An observation that Mnemiopsis actually preys on Penilia to a lesser extent than on copepods is interesting. This could change the ecosystem from one dominated by copepods to one dominated by cladocerans and comb jellyfish. It is likely that it also has effects on phytoplankton, with greater occurrence of larger diatoms and fewer pico- and nanoplankton. But for the present these are only speculations, which require research of a more fundamental nature into the trophic cascades which we are now observing.

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