Grey’s Beach Infauna Study of Source Dredge Sand off Waikiki
5/14/2008 – 8/29/2008
A beach restoration project is proposed at Grey’s Beach, Waikiki which will use sand dredged from a nearby offshore sand channel. NOAA National Marine Fisheries Service and regulatory agencies require a baseline infauna study to assist in determining if the project may lead to adverse impacts to fisheries. The presence of a balanced indigenous community at the source sand cell and how this community may be recolonized following mining needs to be established. The study described below will provide a quantitative and qualitative analysis of infauna of the proposed sand source and the relationship to surrounding sand areas.
Waikiki beaches are essential to the tourism base and economy of the State of Hawaii and maintaining these beaches in pristine condition is a prime goal for state and federal agencies. Previous efforts to replenish sand for Waikiki beaches have included the addition of source sand from the neighbor islands and Kailua Bay on Oahu, and mansand made from crushed bluestone rock. Sand is gradually transported off the beaches and into shallow waters near shore by currents over time.
To date there has been little interest in the infauna communities associated with source sand and their fate, or temporal benthic recovery of areas used for sand mining. These organisms play an integral role as the energy sources for juvenile and adult fish and invertebrates. Many are feeding on the organic matter in the sand substrate and their tubes and burrowing activity restores porosity and oxygen to the sediments.
The following method will provide sufficient knowledge to allow for estimate diversity and abundance of infauna in sands. Samples will be collected by divers with AECOS Inc. and Sea Engineering and delivered to UH Manoa, Edmondson Hall.
Three replicate samples from within the source sand, and three replicates from two sites outside the source cell would be adequate for community analyses. Nalgene screw cap containers, 1,000ml capacity should be used by AECOS Inc. divers to core down into the upper 5-10 cms of sand. Sand will fill approximately a third or less of the container and 2-4 cms of overlying water. Water that has to be poured out to 2-4 cms depth of a full container should be strained over a 0.25mm sieve and stranded organisms on the sieve backwashed carefully into the sample container with a squeeze bottle. This coring method leaves adequate room in the container for formalin to 10% dilution (with sea water collected in the core sample) to be added post dive. This method collects both the infauna and motile surface dwellers that would be disturbed during sampling and may swim up into the water.
The color and texture of sand samples will be recorded on receipt and a chain of custody form signed and retained at the Wormlab (Edmondson Hall 364). After the samples have been fixed in formalin for a minimum of 48 hrs (stored in Chemical Room, ground floor Edmondson), the infauna will be extracted using sieves to trap specimens 0.5mm or greater using a fume hood (Bilger Hall). Specimens will be transferred to 70% ethanol, sorted to major taxa and stored in glass vials in 70% ethanol. Taxa will be identified using dissecting and compound microscopes and regional keys.
Diversity and density of infauna taxa will be determined for each station from the species present and their abundance. Polychaetes and crustaceans that are known to be part of the diet of fishes will be itemized at a general level (greater than generic level). Data from the stations outside the source sand area will be compared to community structure in the source sand and what is known for the Waikiki area sediments. Infauna may be patchy due to the abundance of a gregarious species at one site that was sampled but not in another. Abundance of a few species does not necessarily indicate an aberrant ecological effect. Recruitment to the source sand area may occur by a few species with pelagic larvae entering from sands outside the source site due to tidal currents, or more slowly by small motile species such as crustaceans and echinoderms and juvenile polychaetes.
Estimates as to how long it will take for the source sand site to recover will be tentative and would depend upon follow up samplings. This is not known for Hawaiian waters and previous source sand infauna studies have not followed up with subsequent samplings.[/vc_column_text][/vc_column_inner][vc_column_inner width=”1/3″][vc_column_text]