The unnatural history of Kāne'ohe Bay: coral reef resilience in the face of centuries of anthropogenic impacts

Abstract

Kāne'ohe Bay, which is located on the on the NE coast of O'ahu, Hawai'i, represents one of the most intensively studied estuarine coral reef ecosystems in the world. Despite a long history of anthropogenic disturbance, from early settlement to post European contact, the coral reef ecosystem of Kāne'ohe Bay appears to be in better condition in comparison to other reefs around the world. The island of Moku o Lo'e (Coconut Island) in the southern region of the bay became home to the Hawai'i Institute of Marine Biology in 1947, where researchers have since documented the various aspects of the unique physical, chemical, and biological features of this coral reef ecosystem. The first human contact by voyaging Polynesians occurred at least 700 years ago. By A.D. 1250 Polynesians voyagers had settled inhabitable islands in the region which led to development of an intensive agricultural, fish pond and ocean resource system that supported a large human population. Anthropogenic disturbance initially involved clearing of land for agriculture, intentional or accidental introduction of alien species, modification of streams to supply water for taro culture, and construction of massive shoreline fish pond enclosures and extensive terraces in the valleys that were used for taro culture. The arrival by the first Europeans in 1778 led to further introductions of plants and animals that radically changed the landscape. Subsequent development of a plantation agricultural system led to increased human immigration, population growth and an end to traditional land and water management practices. The reefs were devastated by extensive dredge and fill operations as well as rapid growth of human population, which led to extensive urbanization of the watershed. By the 1960's the bay was severely impacted by increased sewage discharge along with increased sedimentation due to improper grading practices and stream channelization, resulting in extensive loss of coral cover. The reefs of Kāne'ohe Bay developed under estuarine conditions and thus have been subjected to multiple natural stresses. These include storm floods, a more extreme temperature range than more oceanic reefs, high rates of sedimentation, and exposure at extreme low tides. Deposition and degradation of organic materials carried into the bay from the watershed results in low pH conditions such that according to some ocean acidification projections the rich coral reefs in the bay should not exist. Increased global temperature due to anthropogenic fossil fuel emmisions is now impacting these reefs with the first "bleaching event" in 1996 and a second more severe event in 2014. The reefs of Kāne'ohe Bay have developed and persist under rather severe natural and anthropogenic perturbations. To date, these reefs have proved to be very resilient once the stressor has been removed. A major question remains to be answered concerning the limits of Kāne'ohe Bay reef resilience in the face of global climate change.

Keywords: Climate change; Coral reefs; Corals; Eutrophication; Hawaii; Kāneʻohe Bay; Natural history; Reef resilience.

Figure 1. Dredging and filling areas in Kāneʻohe Bay, Oʻahu Hawaiʻi.

Dredged areas (red) and filled areas (black) in Kāneʻohe Bay on the island of Oʻahu. Modified after Maragos 1972. Photo Credit: Quickbird Digital Globe.

Figure 2. Algal dominance.

Photographs of Dictyospheria cavernosa over growth of Porites compressa colony at a long term monitoring site in Kāneʻohe Bay in 1999 and 2000. Photographs by PL Jokiel.

Figure 3. Coral bleaching and recovery from 2014 event.

Photographs of the reef flat on the fringing reef surrounding Moku o Loʻe (Coconut Island) during the second large scale bleaching event in Kāneʻohe Bay in October 2014 and December 2014. Photographs by KD Bahr.

Figure 4. Kāneʻohe Bay reef conditon over time.

Changes in coral reefs condition (percent coral cover) on shallow slopes (<2 m) of fringing and patch reefs during the Polynesian Era (1250–1778), the Western Era (1778–2015), and the Future Era (2015–2040). Percent coral cover during the Western Era is based on best available quantitative data. The Polynesian Era data is modified from Fig 4E in Kittinger et al., 2011. Future Era coral cover is estimated by the COMBO business as usual (red) (modified after Fig 7 in Buddemeier et al. (2008)) and by including coral adaptive responses (blue) RCP 6 with 60 year rolling climatology (modified after Fig. 3A in Logan et al. (2013)) in Kāneʻohe Bay.

Figure 5. Reef response to major impacts during Western Era.

Changes in percent coral cover in response to major impacts during the Western Era (1778–2015) based on best available data. A kite diagram weights influence of impact on the coral reef by thickness of the line over time. Perturbations of freshwater kills (blue) and bleaching events (red) occurrences are indicated by arrows in the timeline.