Richard W. Grigg

Recolonization of Hermatypic Corals on Submerged Lava Flows in Hawaii

Recovery time for recolonization of coral communities on submerged lava flows in Hawaii may largely depend on exposure to sea and swell. In exposed areas, recovery time (in terms of number of species, per cent cover, and diversity) of areas decimated by lava and perhaps similar disturbances appears to be about 20 yr. At sheltered stations, more than 50 yr may be required for complete recovery. This difference in recovery time is apparently related to differences between undisturbed coral communities at exposed and sheltered stations: in exposed areas succession seems to be constantly interrupted, resulting in pioneer stages, whereas at sheltered stations reefs are more fully developed in terms of percent cover and thickness, and are thus closer to being climax coral communities; they therefore require more time for full recovery. The data, although meager, indicate that diversity increases during succession, but attains a peak value before climax is reached. The apparent decline in diversity as climax is approached may be due to interspecific competition for space, which leads to resource monopolization. Comparison of community structurewith more tropical coral reefs reveals that this pattern may apply only to physically controlled communities.

Holocene coral reef accretion in Hawaii: a function of wave exposure and sea level history

Abstract In the high Hawaiian Islands, significant accretion due to coral reef growth is limited by wave exposure and sea level. Holocene coral growth and reef accretion was measured at four stations off Oahu, Hawaii, chosen along a gradient in wave energy from minimum to maximum exposures. The results show that coral growth of living colonies (linear extension) at optimal depths is comparable at all stations (7.7–10.1 mm/y), but significant reef accretion occurs only at wave sheltered stations. At wave sheltered stations in Hanauma Bay and Kaneohe Bay, rates of long term reef accretion are about 2.0 mm/y. At wave exposed stations, off Mamala Bay and Sunset Beach, reef accretion rates are virtually zero in both shallow (1 m) and deeper (optimal) depths (12 m). At wave sheltered stations, such as Kaneohe Bay and Hanauma Bay, Holocene reef accretion is on the order of 10–15 m thick. At wave exposed stations, Holocene accretion is represented by only a thin veneer of living corals resting on antecedent Pleistocene limestone foundations. Modern coral communities in wave exposed environments undergo constant turnover associated with mortality and recruitment or re-growth of fragmented colonies and are rarely thicker than a single living colony. Breakage, scour, and abrasion of living corals during high wave events appears to be the major source of mortality and ultimately limits accretion to wave sheltered environments. Depth is particularly important as a modulator of wave energy. The lack of coral reef accretion along shallow open ocean coastlines may explain the absence of mature barrier reefs in the high Hawaiian Islands.

POPULATION DYNAMICS OF TWO GORGONIAN CORALS

The gorgonian corals, Muricea californica and Muricea fruticosa, co-occur at shallow depths along the western coasts of the Americas. Their northern geographic limit is Point Conception in California; the southern limits are unknown but may be Peru. In California, the depth range of both species is 1-30 m. Their abundances range up to 7.90 and 2.35 colonies/M2, respectively. Both species spawn once each year at a time which corresponds to annual peak temperature, which in California occurs later in the year for deeper segments of the population. Depth-staggered spawning has the effect of extending its duration from 2 to about 6 mo. During the larval stages of both species, mortality exceeds 99%o, whereas after settlement mortality rates are low and nearly constant. Field records show that detachment and abrasion rather than predation are the major sources of mortality for sessile stages. Several lines of evidence suggest that recruitment is generally space limited. In habitats where space is fully occupied, space limitation may regulate recruitment. In such habitats recruitment and mortality of populations of Muricea are nearly constant (i.e. in steady state), and their actual rates of natural increase are close to zero. In contrast, intrinsic rates of natural increase (rmax) are very high: 0.92 for M. californica and 1.45 for M. fruticosa. These enormous rates of potential increase are offset by high rates of larval mortality in the plankton. Although M. californica is significantly more longevous than M.fruticosa, both species share similar patterns of life history. Competition for settlement and living space rarely occurs between the two species; rather, it often takes place with other plants and invertebrates. These interactions may be sufficiently unpredictable for either species of Muricea to develop a competitive advantage over the other and hence may explain their coexistence. Thus, in the absence of significant direct competition between the two species, their similarities in life history may simply represent similar adaptations to the same set of environmental variables. Both species exhibit life history correlates of both K-selection (iteroparity, delayed reproduction, and relative great longevity) and r-selection (high rmax). Life history attributes of a single species may be located at different points along the r-K continuum. In fact, their colonial habit allows for combina- tion of good characteristics of both patterns of selection and may explain the evolutionary success of this type of life history phenomena.

Growth Rings: Annual Periodicity in Two Gorgonian Corals

Comparison of the number of growth rings present in the skeletons of colonies of two gorgonian corals, Muricea californica and M. fruticosa, with estimates of their age based on observed growth rates indicates that the periodicity of ring formation is annual. Data from two other methods used to determine periodicity support this conclusion. The results indicate that the observed growth of these gorgonians decreases constantly as a function of height. Height-age equations are derived for both species. Because the growth rate of both species was found to be variable, counting the rings to estimate age is more accurate than methods based on growth rates.

Paleoceanography of the Tropical Eastern Pacific Ocean

The East Pacific Barrier (EPB) is the most effective marine barrier to dispersal of tropical shallow-water fauna in the world today. The fossil record of corals in the eastern Pacific suggests this has been true throughout the Cenozoic. In the Cretaceous, the EPB was apparently less effective in limiting dispersal. Equatorial circulation in the Pacific then appears to have been primarily east to west and the existence of oceanic atolls (now drowned guyots) in the eastern Pacific probably aided dispersal. Similarly, in the middle and early Mesozoic and late Paleozoic, terranes in the central tropical Pacific likely served as stepping stones to dispersal of tropical shelf faunas, reducing the isolating effect of an otherwise wider Pacific Ocean (Panthalassa).

Coral reefs in an urban embayment in Hawaii: a complex case history controlled by natural and anthropogenic stress

The effects of natural and anthropogenic stress need to be separated before coral reef ecosystems can be effectively managed. In this paper, a 25 year case history of coral reefs in an urban embayment (Mamala Bay) off Honolulu, Hawaii is described and differences between natural and man-induced stress are distinguished. Mamala Bay is a 30 km long shallow coastal bay bordering the southern (leeward) shore of Oahu and the city of Honolulu in the Hawaiian Islands. During the last 25 years, this area has been hit by two magnitude 5 hurricane events (winds > 240 km/h) generating waves in excess of 7.5 m. Also during this period, two large sewer outfalls have discharged up to 90 million gallons per day (mgd) or (360 × 106 L/day) of point source pollution into the bay. Initially the discharge was raw sewage, but since 1977 it has received advanced primary treatment. Non-point source run-off from the Honolulu watershed also enters the bay on a daily basis. The results of the study show that discharge of raw sewage had a serious but highly localized impact on shallow (∼10 m) reef corals in the bay prior to 1977. After 1977, when treatment was upgraded to the advanced primary level and outfalls were extended to deep water (> 65 m), impacts to reef corals were no longer significant. No measurable effects of either point or non-point source pollution on coral calcification, growth, species composition, diversity or community structure related to pollution can now be detected. Conversely the effects of hurricane waves in 1982 and 1992 together caused major physical destruction to the reefs. In 1982, average coral cover of well-developed offshore reefs dropped from 60–75% to 5–15%. Only massive species in high relief areas survived. Today, recovery is occurring, and notwithstanding major future disturbance events, long-term biological processes should eventually return the coral ecosystems to a more mature successional stage. This case history illustrates the complex nature of the cumulative effects of natural and anthropogenic stress on coral reefs and the need for a long-term data base before the status of a coral reef can be properly interpreted.

Uplift caused by lithospheric flexure in the Hawaiian Archipelago as revealed by elevated coral deposits

The presence of definitive indicators of sea level such as corals found in growth position, coralline beach deposits, wave-cut notches and wave-eroded terraces, all at elevations above known levels of eustatic sea level during the past 500 ka, provides strong evidence that uplift has occurred in the central high Hawaiian Islands. Lithospheric flexure is proposed as the mechanism for uplift. The modelled wavelength and amplitude of flexure roughly correspond to the distance and height of elevated coral-rich deposits. A trend of increasing age of coralline beach deposits with elevation on Oahu and Molokai adds support to the uplift hypothesis. Data and observations presented in this paper are inconsistent with a tsunami origin for most elevated deposits on Lanai, Molokai and Oahu. A giant wave origin may only apply to scattered and localized deposits concentrated in low-lying gulches (< 100 m), such as those on Lanai.

Critical Depth for the Survival of Coral Islands: Effects on the Hawaiian Archipelago

Coral islands drown when sea level rise exceeds the maximum potential of coral reefs to grow upward (about 10 millimeters per year). During the Holocene transgression (18,000 years ago to present) sea levels rose at rates of up to 10 to 20 millimeters per year, and most coral island reefs situated deeper than a critical depth of30 to 40 meters below present day sea level drowned. Coral islands that did not drown during the Holocene transgression apparently all developed on antecedent foundations shallower than critical depth. During low stands in sea level during the Pleistocene, these islands were elevated and subject to subaerial erosion. Today, in the Hawaiian Archipelago, the depth of drowned banks is inversely related to summit area; smaller banks are progressively deeper, evidently because of erosional truncation during low sea level stands. Bank summit area may therefore be an important factor determining the failure or success of coral islands.

Paleoceanography of coral reefs in the Hawaiian-Emperor Chain — revisited

Abstract. This paper is a review of the present knowledge of coral reef ecology and paleoecology in the Hawaiian-Emperor Chain during the last 70 Ma. Research on fossil coral deposits throughout the H-E Chain has played a major role in producing information concerning the subsidence, uplift and drowning of individual islands, as well as the paleocirculation of the North Pacific Ocean during Tertiary time. The origin of the Hawaiian Islands and seamounts over the Hawaiian hotspot and their subsequent subsidence and transport by plate motion to the north and northwest are reviewed and new data concerning uplift of the central high Hawaiian Islands caused by lithospheric flexure and the effect of sea level change on the geological evolution of individual islands are presented. The Darwin Point, where atolls drown to form guyots, is redefined in dynamic terms as a function of climate and sea level history. The absence of coral reefs during the first half of Hawaiian history is evidence for a major change in ocean circulation in the north Pacific about 35 million years ago during the Middle Tertiary.

Age Structure of a Longevous Coral: A Relative Index of Habitat Suitability and Stability

Data presented in this paper illustrate how differences in age structure between isolated populations of some longevous benthic invertebrates may be useful in interpreting both the relative suitability and stability of habitats in which they occur. Both measures are species specific. The suitability of a habitat for any species is most easily judged by measures of relative longevity. Habitat stability is defined as the variability between age classes of the population at a single locality. For Muricea californica, this was calculated by fitting several monotonically decreasing functions to the age frequency distributions of populations in different habitats and comparing their standard deviations. The results show that both suitability and stability of the habitat of M. californica are related to features of the substratum, although the most suitable habitats were not necessarily the most stable. The curve-fitting method, while lacking some precision, permits quantitative contrasts between habitats. Also, past changes in a habitat that occurred during the life span of a population may be evident by examining age structure, as in the case of the apparent effect of sewage on the Palos Verdes population of M. californica, an example of environmental hindcasting. If the species selected for study is a dominant member of the community in which it is found, or if it is a keystone or foundation species (Paine 1969; Dayton 1972), then its age structure may also reflect the environmental suitability and stability of many other associated or dependent species. In this sense, the index for such a species is not necessarily species specific but, rather, may be applied more broadly at the community level.