MEPS 226:103-114 (2002)  -  doi:10.3354/meps226103

Maintenance of species dominance through pulse disturbances to a sessile marine invertebrate assemblage in Port Shelter, Hong Kong

Emma L. Johnston1,*, Michael J. Keough1, Pei-Yuan Qian2

1Department of Zoology, University of Melbourne, Victoria 3010, Australia
2Department of Biology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, SAR
*Present address: Centre for Marine and Coastal Studies, School of Biological Sciences, University of New South Wales, Sydney 2052, Australia. E-mail:

ABSTRACT: Pollution events can be considered as disturbances to an ecosystem and have the potential to be an important structuring force within assemblages of plants and animals. Our understanding of pollution impacts will benefit from the investigation of important aspects of these disturbances such as their timing and frequency. We investigated the effect of pulse copper pollution events on the development of hard-substrate invertebrate assemblages in Port Shelter, Hong Kong. Using a field dosing technique, we created single or repeated copper pollution events of less than 100 µg l-1 close to settlement plates. In short-term experiments, pulses were delivered in either the first or the second week of substrate immersion. For longer term experiments, pulses were delivered every 2, 4, 8 or 16 wk for the duration of the 16 wk experiments and the assemblages allowed to develop under these dosing regimes. Pulse pollution events had a dramatic effect on densities of the dominant colonizer and space occupier in short-term experiments, Hydroides elegans. The impact was observed as a change in the size structure of the population with large organisms eliminated close to copper sources while new recruits were increased in densities in the same area. Reductions in densities of bryozoans and barnacles were also observed, while a feather hydroid doubled in densities on plates exposed to copper pulses. Young assemblages recovered rapidly from the impact of a copper pulse and reduced impacts were observed from a copper pulse in Week 1 of the experiments compared to exposure in Week 2. In longer-term experiments, copper pulses also dramatically altered assemblages predominantly through a direct negative effect on the densities of large competitively superior colonial and solitary ascidians. In the absence of a regular pollution event, colonial and solitary ascidians and sponges were able to establish, grow-over and/or push out the smaller, more brittle H. elegans. These good competitors were unable to establish and grow to any great size on plates exposed to copper pulses every 2 or 4 wk and under the influence of repeated pulse copper exposures H. elegans maintained its initial dominance. Assemblages exposed every 8 or 16 wk were not significantly different from undosed controls. The results provide information on the type and magnitude of sessile invertebrate responses to transient pollution events as well as an assessment of the time expected to Œassemblage recovery¹ from single or repeated pulse disturbances. This is discussed in relation to the results of identical experiments conducted in temperate climates.

KEY WORDS: Copper · Pulse disturbance · Frequency · Indirect effects · Field experiment · Sessile invertebrates · Recruitment · Disturbance

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