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CRAMP Sediment Analyses

Complex, dynamic processes are involved in the relationship sediment have with coral reefs. Sediment deposits may be influenced by bathymetry, wave regimes, and local currents that affect sediment deposition rates and size fractions. Sediment composition and grain size analyses can provide a characterization of the benthos, supplying information about benthic structure. Determination of sediment composition reflects type and abundance of coral coverage, and substrate. The type of sediment can identify groups of organisms primarily responsible for sediment accumulation. When combined with other means of investigation, sediment analysis can lead to identifying the forcing functions within that habitat.

Sediments also relate to other environmental variables providing a quantitative link to biological parameters such as fish and algal populations. Some physical characteristics of sites can also be defined through sediment analyses including wave exposure and topographical relief. An example would be regions with a high percentage of silt and clay being indicative of relatively low water motion.

Sediments were collected from two depths from each of 29 sites on six of the Main Hawaiian Islands: Hawai’i, Maui, Kaho’olawe, Moloka’i, O’ahu, and Kaua’i. No sediment was found along the 100m transect at the 10m site at Kamalō, Moloka’i. No sediment was collected from the CRAMP site at Hanalei, Kaua’i. Depths ranged from 2m to 13m with the exception of Hokulia, Hawai’i where a permanent site was installed at 23m subsequent to sediment loading from coastal construction in 1999. Grain size and sediment composition were determined for each sample collected.

Approximately 500cc of sediment were collected along the transect at each site and secured in Fisher brand 9 x 18 cm sample bags. Replicate samples were collected at each depth. Sediment grain-size and composition were determined using standard geological methods (Parker, 1983, McManus, 1988, Craft et al., 1991). Standard brass sieves were used to determine size fraction: 2.8 mm, 500 um, 250 um, 125 um, and 63 um (USA Standard Testing Sieve: A.S.T.M.E.-11 specifications). A brass catch pan was used to collect the silt/clay fraction. Four size fractions were determined: gravel, coarse, fines, and silt in accordance with the Wentworth scale (Folk 1974).

To determine the inorganic-organic carbon fraction, 10 grams of sediment were finely ground using a mortar and pestle. Samples were then dried for 10 hrs @ 100^oC, placed in a desiccator and weighed. To remove the organic fraction, samples were burned in a muffle furnace for 12 hrs. @ 500^oC, placed in a desiccator and weighed. For removal of carbonate material, samples were placed in a muffle furnace for 2 hrs. @ 1000^oC, cooled in a desiccator and weighed. The percent loss on ignition was calculated from this data.

Sediment Composition

Loss on Ignition (LOI)

Sediment from all three sites at both depths in Kāne’ohe Bay and at Hakioawa, Kaho’olawe contain a high percentage of organic material. These outliers have between 7% and 17% organic matter, exhibiting the highest values in the state. All other sites range from a low of 0.18% at the 3m depth at Ka’apuna, Hawai’i to a high of 5.12% at the 3m depth at Laupāhoehoe, Hawai’i (Table 1).

• Hawai’i—The highest organic percentage on the island of Hawai’i is at
  Laupāhoehoe 3m (5.12%) and at  Kawaihae 3 & 10m (3.75% & 3.61%). Kawaihae has had extensive harbor development and modification that has interrupted long-shore sediment transport.

• Maui—Three sites on Maui have relatively high levels of organic material: Honolua North 3m (3.60%) and Honolua South 3m (4.68%), Puamana 3m & 10m (3.91% & 4.82%), and Olowalu 10m (3.79%).

• Kaho’olawe—Both depths at Hakioawa, Kaho’olawe have a high percentage of organic matter relative to the rest of the state (11.36% & 11.46%). Kaho’olawe has a history of sediment loading due to devegetation from feral goats and bombing target practice. Subsequent animal removal occurred and revegetation projects are currently taking place.

• Moloka’i—Of the three Moloka’i sites, Kamalō 3m contains the least organic material (2.07%). This is the site on Moloka’i least impacted by sediment loading due to urbanization, and devegetation.

• O’ahu—Sites within Kāne’ohe Bay had the highest organic content in the state. Sites are monitored in all three sectors of the Bay. The South Bay site, located on Moku o lo’e has the lowest organic content of the three sites (2m- 9.92%, 8m-7.35%). The site in the Central Basin located at He’eia has considerably more organics (2m-13.40%, 8m-14.55%) Ka’alaea located in the North Bay has the highest organics in the state (2m-17.13%, 8m-14.51%). Kāne’ohe Bay has an extensive history of dredging and sewage discharge with considerable urbanization in the surrounding watershed.

• Kaua’i—All sites on Kaua’i are found to be relatively similar in organic content of sediments.

H₂CO₃

Sediments at 19 depths contain over 90% calcium carbonate. These sites are found on all of the islands sediments were collected from. High H₂CO₃ is consistent with high coral cover. Ka’apuna, Hawai’i is anomalous due to extremely low amounts of H₂CO₃ at both depths (0.18% & 0.31%). This relatively recent 1950’s lava flow is low in coral cover (7.7%) ranking 50th among the 60 reefs surveyed. The sediments from this site contain low levels of organic material and H₂CO₃ and extremely high levels of terrigenously based basalt (Table 1).

• Hawai’i—Kawaihae have high calcium carbonate material at both depths (3m-93.12%, 10m-93.34%). La’aloa 3m (87.27%) and 10m (88.74%), and Leleiwi 10m (88.46%) were also high in H₂CO₃.

• Maui—Three sites have greater than 90% H₂CO₃: Papa ̒ula 3m (94.92%) and 10m (94.33%), Ma ̒alaea 3m (91.14%) and 10m (92.89%), and Kanahena Point 10m (92.63%). Sediments from Olowalu 10m (28.22%) and Puamana 3m (35.05%) have the least H₂CO₃ of the Maui sites.

• Kaho’olawe—Hakioawa has one of the lowest percentages of H₂CO₃ in the state (3m-17.2% and 10m-27.5%). Although H₂CO₃is relatively low, coral cover at this site is moderately high relative to other sites.

• Moloka’i—Sediments from all depths at all sites are high in H₂CO₃.

• O’ahu—The two sites at Kahe: Pili o Kahe 3m (93.59%) and Kahe Point 3m (93.11%) and the two depths at Pūpūkea 3m (96.03%) and 10m (95.68%) have the highest levels of H₂CO₃on O’ahu. All other O’ahu sites are in protected bays (Hanauma and Kāne’ohe).

• Kaua’i—Ho’ai 3m (93.24%) and 10m (92.63%), Nu‘alolo Kai 10m (92.60%), and Miloli’i 3m (89.87%) and 10m (89.68%) have relatively high amounts of H₂CO₃.

Terrigenously based materials

Five sites have greater than 50% materials other than organics and H₂CO₃. These are comprised mainly of basalt. Not surprisingly, Ka’apuna, Hawai’i the most recent lava flow of the selected sites has the highest percentage in the state: 3m (99.68%) and 10m (98.59%). Other sites with high percentages of basalt include Hakioawa, Kaho’olawe 3m (71.44%) and 10m (61.03%), and Laupāhoehoe, Hawai’i 10m (83.85%). While sites on Hawai’i the youngest island have a sediment composition high in basalt, sites on Kaua’i, the oldest of the main Hawaiian islands have very low levels.

Table 1: Sediment Composition

Statistical Analyses

A scatterplot of sediment composition groups the sites having similar organic composition together in ordination space (Figure 1). These include Hakioawa, Kaho’olawe and all Kāne’ohe Bay sites. The extreme outlier, Ka’apuna, Hawai’i is low in both organics and calcium carbonate, consisting mainly of basalt. Most of the other sites group together, consisting of high H₂CO₃ and between 2% and 4% organic material. Another cluster of sites have lower H₂CO₃ than the main grouping and exhibit diverse exposure, depth, and latitudinal gradients.

Figure 1 Sediment Composition

Figure 1 Sediment Composition

Sediment Grain Size

Gravel (>2.8mm) Coarse (>500um) Fines (>63um) Silt (<63um). Extremely large pieces were removed before sorting to reduce variability and eliminate overweighting of some samples by a single piece of material. Most samples from protected areas, harbors, and bays were found to have low levels of large material and high levels of silt. The 6 Kāne’ohe Bay samples have the highest percentages of silt in the state (up to 63.06%) followed by Hakioawa at Kaho’olawe (14.19%). Sites high in silt size fractions are also found to be high in organic matter. Conversely, many north facing sites that are exposed to periodic storm surf and strong currents were found to have very low levels of fine grained particles and corresponding high percentages of large material. Sediments from north facing sites contain the highest statewide percentages of gravel: Pūpūkea. O’ahu 8m (94.35%), Miloli’i, Kaua’i 3m (97.15%), and Nu‘alolo Kai Kaua’i10m (98.51%) (Table 2).

• Hawai’i—Evaluating the just the extremes, gravel dominated sediments at Ka’apuna 10m (95.90%) and Hokulia 23m (87.80%). The highest percentage of silt on this island occurs at Laupāhoehoe 3m (3.19%) and at Kawaihae 10m (2.62%).

• Maui—Following the trend of high organic material, Puamana 13m (3.92%) and Honolua South 3m (2.24%) have the highest silt fractions on Maui. The largest grain sizes are found at Papaula Point (79.27%, 73.72%) and Kanahena Bay 3m (82.54%).

• Kaho’olawe—Both depths at Hakioawa, Kaho’olawe have a high percentage of silt relative to the rest of the state (9.19% & 14.19%). This is to be expected with Kaho’olawe’s history of sedimentation due to devegetation and bombing target practice

• Moloka’i—Of the three Moloka’i sites, Pala’au has the highest large-and fine-grain percentages. The protected 1m site, situated on a reef flat adjacent to a deeper channel with a mangrove dominated shoreline has 8.46% silt in the sediments. Contrary to the shallow site, the deeper, offshore 10m site has sediments dominated by gravel (96.6%).

• O’ahu—Sediments within Kāne’ohe Bay have the highest silt content in the state. The site in the Central Basin located at He’eia (2m-55.54%, 8m-59.90%) and the site in the North Bay at Ka’alaea (2m-30.73%, 8m-63.06%) are mainly dominated by very fine-grain material. Extensive dredging, sewage discharge, urbanization, bathymetry, and circulation patterns have led to conditions within the bay that are atypical of the rest of the state. The sediments at the north exposed site at Pūpūkea is heavily influenced by gravel (95.81%, 94.35%).

• Kaua’i—All sites on Kaua’i are found to be relatively similar having very low percentages of silt relative to the rest of the state. Sediments from north facing sites are dominated by gravel.

Table 2 Sediment Grain Size