Cynthia L. Hunter

Ecological impact of a fresh-water “reef kill” in Kaneohe Bay, Oahu, Hawaii

Storm floods on the night of December 31, 1987 reduced salinity to 15‰ in the surface waters of Kaneohe Bay, resulting in massive mortality of coral reef organisms in shallow water. A spectacular phytoplankton bloom occurred in the following weeks. Phytoplankton growth was stimulated by high concentrations of plant nutrients derived partially from dissolved material transported into the bay by flood runoff and partially by decomposition of marine organisms killed by the flood. Within two weeks of the storm, chlorophyll a concentrations reached 40 mg m-3, one of the highest values ever reported. The extremely rapid growth rate of phytoplankton depleted dissolved plant nutrients, leading to a dramatic decline or “crash” of the phytoplankton population. Water quality parameters returned to values approaching the long-term average within 2 to 3 months. Corals, echinoderms, crustaceans and other creatures suffered extremely high rates of mortality in shallow water. Virtually all coral was killed to depths of 1–2m in the western and southern portions of the bay. Elimination of coral species intolerant to lowered salinity during these rare flood events leads to dominance by the coral Porites compressa. After a reef kill, this species can eventually regenerate new colonies from undifferentiated tissues within the “dead” perforate skeleton. Catastrophic flood disturbances in Kaneohe Bay are infrequent, probably occurring once every 20 to 50 years, but play an important role in determination of coral community structure. The last major fresh water reef kill occurred in 1965 when sewage was being discharged into Kaneohe Bay. Coral communities did not recover until after sewage abatement in 1979. Comparison between recovery rate after the two flood events suggests that coral reefs can recover quickly from natural disturbances, but not under polluted conditions.

Field biology of Halimeda tuna (Bryopsidales, Chlorophyta) across a depth gradient: comparative growth, survivorship, recruitment, and reproduction

Growth, survivorship, recruitment, and reproduction of Halimeda tuna, a dominant green alga in many reef systems of the Florida Keys, were monitored at a shallow back reef (4–7m) and deep reef slope (15–22 m) on Conch Reef. Despite lower light intensities and similar grazing pressures, amphipod infestations, and epiphyte loads at both sites, the deeper site exhibited significantly higher growth rates in summer months over a 4-year period than found for the shallow population, possibly because of higher nutrient levels at depth and photoinhibition of shallow plants. Sexual reproductive events occurred simultaneously across the entire reef, with up to 5% of the population at both sites developing gametangia. New upright axes formed from zygotes, asexual fragmentation, or vegetative runners. Plants appear to have persistent basal stumps that survive harsh environmental conditions, even if upright, photosynthetic axes are removed. Sexual reproduction and ‘smothering’ by epiphyte overgrowth are hypothesized to be two causes of death for individuals.

Contaminants in oysters in Kaneohe Bay, Hawaii

Despite past and present concerns about the toxicity and persistence of various environmental contaminants (heavy metals and pesticides), relatively few studies have documented their concentrations in tropical or sub-tropical marine ecosystems. In this investigation, a ‘mussel watch’ approach was applied in Kaneohe Bay, Hawaii, to assess the present levels of potential contaminants in the Pacific oyster, Crassostrea gigas. Geographical gradients of these contaminants were observed and related to what is known about past and present inputs. Concentrations of lead, copper, chromium and zinc were elevated in oyster tissues near stream mouths in the southern watersheds of Kaneohe Bay. Dieldrin and chlordane concentrations in oysters from one of these sites exceeded the US Environmental Protection Agencys screening levels to protect human health, and were much higher than in oysters from the east and Gulf coasts of the temperate mainland USA.

Variability in the ecophysiology of Halimeda spp. (Chlorophyta, Bryopsidales) on Conch Reef, Florida Keys, USA

The photosynthetic performance, pigmentation, and growth of a Halimeda community were studied over a depth gradient on Conch Reef, Florida Keys, USA during summer–fall periods of 5 consecutive years. The physiology and growth of H. tuna (Ellis & Solander) Lamouroux and H. opuntia (L.) Lamouroux on this algal dominated reef were highly variable. Maximum rate of net photosynthesis (Pmax), respiration rate, and quantum efficiency (α) did not differ between populations of either species at 7 versus 21 m, even though the 21‐m site received a 66% lower photon flux density (PFD). Physiological parameters, as well as levels of photosynthetic pigments, varied temporally. Pmax, saturation irradiance, compensation irradiance, and growth were greatest in summer months, whereas α, chl a, chl b, and carotenoid concentrations were elevated each fall. Halimeda tuna growth rates were higher at 7 m compared with 21 m for only two of five growth trials. This may have arisen from variability in light and nutrient availability. Individuals growing at 7 m received a 29% greater PFD in August 2001 than in 1999. In August 1999 and 2001 seawater temperatures were uniform over the 14‐m gradient, whereas in August 2000 cold water regularly intruded upon the 21‐m but not the 7‐m site. These results illustrate the potentially dynamic relationship between nutrients, irradiance, and algal productivity. This suggests the necessity of long‐term monitoring over spatial and temporal gradients to accurately characterize factors that impact productivity.