Hilo Bay Watershed Advisory Group members attended a one-day teacher’s workshop focusing on a new curriculum that introduces “The Mystery of the Mostly Missing Reefs” to intermediate level students.  Class work was conducted at the Mokupapapa Discovery Center prior to a field trip to Mokuola (Coconut Island) where participants engaged in sampling and analyzing the nearby waters for different properties as well as performing visual observations.  Photos of the event can be found by clicking here.  The curriculum which is available to middle school teachers was developed by the Pacific American Foundation through a grant from the US Department of Education Native Hawaiian Education Act and from Tesoro Hawaii Corporation through the National Wildlife Foundation.

Background Information

The corals within Hilo Bay are actually very limited in number and consist of mainly “recruits” – small, visible colonies of corals.  Just within the breakwall, recruits of brown and blue Montipora sp. and some Proites sp. are visible.  However, they are not considered established coral reefs because they are not structures made from the skeletons of soft-bodied coral animals or polyps in warm waters, but rather are independent recruits growing on lava rock.  The Hilo Bay breakwall was constructed on Blonde Reef, a high energy reef with various plate and encrusting corals, numerous channels, and spurs. (Coney, 2009)

A variety of hydrologic factors contribute to the relative scarcity of corals within Hilo Bay.  The primary reason for this scarcity is the high volume of fresh water entering the bay through surface and groundwater sources.  Sediment borne by surface flows also contributes to a decrease in available light for the zooxanthellae symbionts in corals.  Corals such as Montipora sp. appear to be resilient to stresses such as sedimentation and salinity changes, hence their appearance within the breakwall.  (Gulko, 1998)

All corals, worldwide, have relatively specific environmental requirements.  Most reefs develop in waters with mean temperatures of 23-25°C.  Both depth and light are important limiting factors for coral growth.  The amount of light affects the ability of zooxanthellae to photosynthesize and this, in turn, reduces the coral’s ability to secrete calcium carbonate – the primary constituent in coral skeletons.  In general, coral development is greater in areas of strong wave action which contributes to a source of fresh, oxygenated seawater.

Two other characteristics that impact coral reef growth are salinity and sedimentation, both significant factors in the Hilo Bay estuarine environment.  Sediments contribute to the Bay’s turbidity, reducing the light needed for photosynthesis.  (Nybakken, 1993)

Invasive algae can also cause serious problems.  Two such invasive species are prevalent on Hawaii Island, Acanthophora spicefera and Gracilaria salicornia.  As with all invasive algal species, displacement of native limu and dense growth patterns can negatively affect coral growth.  Excess nutrients such as nitrates and phosphates from surface and groundwater sources can also contribute to macroalgal blooms and phytoplankton blooms; that, in turn, impact the turbidity of the water and subsequently coral development.

“In a balanced, healthy reef ecosystem, corals and coralline algae are the dominant species.  The fleshy limu are less dominant though they play an important role at the base of the food chain by supporting invertebrates and herbivorous fish that feed on them.  When nutrients such as nitrates and phosphates from human sewage and lawn and garden fertilizers wash onto the reef, conditions favor rapid growth of limu.  Overfishing also contributes to limu growth by removing herbivorous fish which help control its growth.  Combining overfishing with excessive nutrients from human activities sets the stage for environmental problems.” (Aloha ‘Aina 2009)

For more information regarding the curriculum, contact Maura O'Connor at mauraoc@hawaii.rr.com or visit www.thepaf.org.