in the marine habitat and biota of Pelekane Bay, Hawai’i
Results indicate that major changes have taken place in the marine biota of Pelekane Bay since 1976. Although the absolute areas of different habitat types within the bay appear to have been stable, there have been striking declines in the abundance of all plants and animals associated with major changes in species diversity and composition. Although the causes of these changes cannot be ascertained directly from this study, it seems likely that the community is changing in response to long-term sedimentation stress associated with chronic terrestrial run-off and reduced ocean circulation in Pelekane Bay.
Changes in marine biota
With the exception of the coralline algae Porolithon onkodes and P. gardeneri, the brown alga Padina japonica, and sparse patches of filamentous "turf" algae, marine plants have largely disappeared from Pelekane Bay. Surveys by Ball (1977) in 1976 found 13 species present, only two of which were abundant. Ball (1977) concluded that the low plant diversity was due to high rates of sedimentation coupled with reduced water motion associated with the Kawaihae Harbor revetment. Based on the results of this study, it would appear that these factors have caused continued declines in algal abundance and diversity over the intervening 20-year period. The only marine plant present in 1996 and listed in Ball (1977) in 1976 was Porolithon onkodes, which was described as moderately abundant on coral rubble. In this study live P. onkodes was rare on coral rubble and accounted for only 0.6% of substratum cover. In addition, dead P. onkodes accounted for 0.8% of the substratum cover, suggesting that it has also experienced high mortality.
The abundance and diversity of invertebrates also experienced major declines between the two surveys. Chaney et al. (1977) described 106 species of invertebrates associated with the soft sediments, coral rubble and patch reefs of Pelekane Bay in 1976. In and amongst coral rubble they found numerous species of polychaete worms, crustaceans, gastropods, sea cucumbers and sea urchins. During surveys in 1996, these same coral rubble areas were largely bare, with occasional Alpheus shrimp burrows, vermetid gastropods, shore crabs, and several unidentifiable worm burrows. Moreover, the infauna, which consisted of several species of polychaetes, and Alpheus and Callinassid shrimp burrows in 1976, were greatly reduced in 1996. Although several unidentifiable worm burrows and Alpheus shrimp burrows were commonly seen during this study, repeated samples of sediment throughout the Bay contained few macroscopic organisms. Anoxic conditions were often encountered within several centimeters of the surface, perhaps in response to chronic accumulations of sediment. Thus, the overall diversity of invertebrates declined 82% between the 20 year period of the two studies.
On patch reefs the abundance and diversity of corals also experienced dramatic changes. Of the ten coral species surveyed in 1976, only five were seen in 1996. In contrast, 300 m north of Pelekane Bay near the Kawaihae small-boat harbor, 11 species of corals are found (US Fish and Wildlife Service, 1993). Moreover, the abundance of living corals declined from 44% to 6.7% cover and there was a corresponding increase in the abundance of dead coral. Interestingly, mortality appears to have been the highest in branched corals such as Porites compressa and Pocillopora meandrina, which experienced 82% and 89% percent declines in abundance, respectively. The more massive coral, Porites lobata, experienced a smaller 65% decline. This pattern is in contrast to that observed in several studies which have demonstrated lower mortality in branching species due to their natural ability to eliminate sediments (Rodgers, 1983, 1990). This pattern suggests that additional physical factors in Pelekane Bay, such as increased temperatures and lower salinities, may also be contributing to mortality.
Marked declines in invertebrate abundance are not suprising considering the strong negative effects chronic sedimentation has on invertebrate growth and reproduction. Among corals, declines in both abundance and species diversity are common response to increased rates of sedimentation (e.g., Acevedo and Morelock, 1988). High sediment loads may kill corals by slowing growth rates (Chansang et al., 1992), or smothing, which causes corals to secrete large amounts of mucus to cleanse themselves (Rodgers, 1990). At low rates of sedimentation, this excess mucus secretion causes reduced growth and reproductive output. At higher levels of sedimentation, corals are eventually killed by a combination of smothering and reductions in available energy due to turbid waters. Bak (1979) described a similar response of corals to sediments associated with dredging activities which both increased the turbidity of the water and reduced the amount of circulation. A study by Tissot et al. (1998) also found reduced abundance and diversity of invertebrates and coral growth in dredge holes associated with increased turbidity and reduced circulation. Moreover, sediment cover on the bottom has been shown to prevent larval settlement and thus natural recovery of an area (Hodgson, 1990).
The fish community at Pelekane Bay was also remarkably different between 1976 and 1996. Overall, the abundance of fish declined 68% and there were additional, dramatic changes in fish community structure. Although the 1996 fish community was more diverse in both richness and evenness when compared to the community in 1976, the species composition of the two communities were markedly different. In 1976, the fish community was dominated by relatively few species of fish: goatfish, wrasses, adult parrotfish and several species of surgeonfishes. In 1996, the community was represented by a much more divers mix of juvenile parrotfish, surgeonfishes, snappers, and wrasses. Thus, the overall similarity of the communities between the two surveys was only 19.7%.
Changes in the fish community due not appear to be entirely associated with changes in the plant and invertebrate fish communities. Although there were no significant changes in fish feeding guilds, the incidence of both herbivorous and corallivorous fishes tended to increase despite major declines in seaweeds and corals. Thus, fish may be responding directly to the effects of sedimentation or to other factors which influence fish abundance and diversity such as recruitment, predation and additional human impacts such as fishing. Hypotheses on factors which influence coral reef fish community diversity are controversial but in general support the lottery hypothesis (Sale and Dybdahl, 1975). The lottery hypotheses maintains that coral reef communities are open, non-equilibrium system where chance recruitment events play a major role in structuring abundance patterns (Sale, 1991). As a result, fish communities display low temporal stability and thus display low temporal similarity, a pattern noticed in this study. Thus, the large changes noticed between the fish communities in Pelekane Bay in 1976 and 1996 could be due to a variety of factors.
Recommendations for restoration
It seems highly likely that changes in the marine biota at Pelekane Bay are associated with chronic, long-term sedimentation from the adjacent watershed combined with reduced water circulation in the bay due to the Kawaihae Harbor revetment. Both of these factors were also mentioned in the earlier studies of Ball (1977) and Chaney et al. (1977) as causing significant impacts in 1976. Consequently, a full restoration of Pelekane Bay would require the following changes:
During the process of restoration in would be important to continue monitoring the marine biota in addition to the magtitude and frequency of sedimentation occurring in the area. Changes in the reef community associated with restoration of the Pelekane watershed and subsequent reductions in terrestrial run-off would present a unique and unprecedented opportunity to observe the coupled response of both marine and terrestrial systems to resource management in addition to restoring both an aesthetic, economic and culturally important area.