Background: A History of Seagrass in Maunalua Bay

In the past, Maunalua Bay has supported large meadows of the native seagrass Halophila hawaiiana, as well as corals and a wide range of different organisms that rely on these complex habitats.

Maunalua Bay has historically supported a very diverse community.
Pictured in the center is the seagrass Halophila hawaiiana.

However, due to urbanization, the bay has become increasingly degraded.  During the New Years Eve Flood of 1987, the bay was inundated with muddy sediment that buried near-shore corals, reduced seagrass cover, and provided an ideal habitat for the quick colonization of the invasive green alga Avrainvillea amadelpha, or leather mudweed.  Once it entered the inshore areas of the bay, leather mudweed trapped and held the mud by covering the sand in thick mats, thereby smothering and causing the decline of seagrass meadows throughout the bay.

Leather mudweed (Avrainvillea amadelpha) trapping mud in the bay , creating an inhospitable place for seagrass.

From March 2010 to February 2011, twenty-three acres of Avrainvillea amadelpha have been removed from the inshore areas of Maunalua Bay and it appears that the trapped mud is being flushed from the bay.  As a result of these removal efforts, suitable sandy areas that once supported the seagrass Halophila hawaiiana have been uncovered.
 


A big problem, however, is that H. hawaiiana may be extremely slow to recolonize the cleared areas of the bay on its own due to its limited dispersal capabilities.  Instead of producing seeds, the seagrass mainly reproduces slowly via vegetative growth, or branching from the stem.


H. hawaiiana produces female and male flowers (pictured here) on separate plants.  Even if male and female plants are near each other, they may not be flowering at the same time, thereby preventing pollination and the production of seeds.

Active restoration through the transplantation of H. hawaiiana could potentially accelerate the recovery of seagrass in Maunalua Bay.   In order to reintroduce seagrass communities to cleared areas of the bay, it will be important to know the most effective techniques for transplanting Hawaii’s native seagrass.  While transplantation methods have been developed for other seagrass species, very little such information is currently available for H. hawaiiana.                                          

To determine the best method for restoration, I will test different techniques for actively restoring H. hawaiiana and evaluate the survival and growth rates of the transplanted seagrass using these different techniques.

                                                        Setting up equipment to protect transplanted seagrass

Transplantation Techniques

I will be transplanting seagrass in 8 sandy areas that are free of leather mudweed in Maunalua Bay.  Each of these areas will be considered an experimental "block".  Each block will measure 3.5 m x 2.5 m (or, 11.5 ft x 8 ft) and will contain 6 different transplantation treatments.

                           6 transplantation treatments (shown in blue) will be arranged in each of the 8 experimental blocks.

The treatments are designed to test the effect of adding increasing levels of protection on the survival and growth rates of the transplanted seagrass.

Conceptual illustration of the 6 transplantation treatments

*  In Treatment 1, the seagrass will be surrounded by a low cage secured over the plot. These cages are expected to both exclude larger herbivores and limit bioturbation (mixing of sand by animals) by excluding fish that forage for food near seagrass.  Also, the sediment around the transplants will be stabilized by plastic mesh.  This is to prevent sediment from leaving the plot and to prevent the new transplants from getting buried by shifting sand.
                                                      

                                                        
                             Photo of Treatment #1, consisting of both a mesh cage and mesh attached to the seafloor

*  In Treatment 2, the seagrass will be caged but NO plastic mesh will be added for stabilization.

                                     
                                  Photo of Treatment #2, consisting of only a protective cage


* Treatment 3 will be a cage control for light.  This treatment will demonstrate whether or not the mesh cages harm the seagrass by reducing the amount of light available to perform photosynthesis.

             Photo of Treatment #3, consisting of a cage top with no sides.  This device will not      protect the seagrass from herbivores or fishes foraging in the sand, but it can block light in a manner similar to complete cages.

* In Treatment 4, only mesh will be added to the seafloor to stabilize the sediments.

                                              

Photo of Treatment #4, consisting only of mesh attached to the seafloor

* In Treatment 5, neither mesh nor caging will be added to protect the transplanted seagrass.

 * Treatment 6 will be a bare sediment control plot.  No seagrass will be transplanted in this area,   allowing it to serve as a reference and show what will colonize in the absence of seagrass.