Littoral macrofauna (secondary) responses to experimental nutrient addition to rocky shore mesocosms and a coastal lagoon
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Kraufvelin P, Christie H, Olsen M
coastal eutrophication, rocky shores, macrofauna, BACIP, mesocosm, whole-ecosystem experiment, Norway, Solbergstrand, Oslofjord, 12 m3
Macrofauna secondary responses to controlled eutrophication within two MARICULT/MAST-III projects, EULITand COMWEB, are compared. EULIT utilises a nutrient gradient established in eight hard bottom mesocosms,whereas the data from COMWEB originate from a whole-ecosystem study – the case of experimental nutrientaddition to Hopavågen lagoon. In both systems, nutrient addition started in May 1998, after initial studies of thebackground macrofauna communities, thus allowing application of Before-After-Control-Impact-Pairs techniques(BACIP). The main objectives have been to investigate the macrofauna responses to eutrophication in littoral rockyshore ecosystems and to evaluate if similar responses could occur in mesocosm and ‘field’ systems, despite theirinherent differences. Apart from a distinct increase in numbers of Littorina littorea L. and some indications ofincreased abundance of the genus Jaera, no significant changes, caused by 2.5 years of nutrient addition, couldbe detected in the mesocosm fauna. It is interesting, however, that these two possibly stimulated animal groupshave two things in common: (1) both belong to the rather few mobile taxa, which are not flushed out of thesystem through the mesocosm outlets, (2) both feed on microalgae and green algae in the upper littoral zone,i.e., algal groups that have shown the clearest response to the nutrient addition. In Hopavågen, 1.5 years ofnutrient addition only caused modest plant and animal responses in the rocky shore ecosystem, although therewas a markedly increased settlement of Mytilus edulis L. at one lagoon site. This increase was probably due toelevated levels of digestible particles (increased phytoplankton production) caused by the nutrient addition. Therewere also some indications of increased production of filamentous algae at this same lagoon site. All in all, bothstudies demonstrate only minor responses to increased nutrient levels within the littoral community (both plantsand animals). One explanation to this may be that longer time-scales are needed in order for clear-cut changes tooccur. Several observations also indicate that wave exposure (significant in both systems) may largely modulatethe impact of increased nutrient load on the structure of littoral communities. Another explanation is the highdegree of stability among littoral macroalgae-dominated communities with internal biological regulation factors(like grazing) possibly being able to counteract effects.
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