CRAMP Rapid Assessment. Sediment Results
Many sheltered areas, harbors and bays have low levels of large material and high levels of fine grain sediment. Many sites that have high silt/clay fractions also have high percentages of organics (rs=0.67).
Five stations were found to have over 50% silt/clay (<63 um). Over 80% of the 91 stations have very small amounts of silt/clay ranging from between 1% and 2% (Figure 1).
Figure 1: Frequency of occurrence of silt/clay fraction for 91 stations (bin size=3%).
Statewide percentages of silt/clay range from 0.1% at Pūpūkea, O‘ahu to 63.1% at Ka‘alaea, O‘ahu. Wider ranges are exhibited for coarse and very coarse sands (500 um-2.8 mm) and fine and very fine sands (63-250 um). Medium sands comprise the smallest percentage of the total at Hanalei, Kaua‘i (0.4%) and the highest percentage at Kahe Point, O‘ahu (82.7%). Fine and very fine sands are lowest at Pūpūkea, O‘ahu (0.3%) and highest at Olowalu, Maui (87.2%). Statewide percentages of the largest size fraction (500 um -2.8 mm) range from 0.1% at Hanalei, Kaua‘i to 98.5% at Nualolo Kai, Kaua‘i, exhibiting the widest range of the four grain-sizes.
Organic Material (Loss on Ignition (LOI))
Sediments from 91 stations at 50 sites were included in the data analysis. Statewide percentage of organics range from 0.2% at Ka’apuna, Hawai‘i to 23.6% at Pelekane Bay, Hawai‘i. The majority of the MHI stations (86%) have organic percentages between 2% and 5% (Figure 2, 3). Several outliers were observed, including Pelekane Bay, Hawai‘i (Figure 4), Kāne‘ohe Bay, O‘ahu (Figure 5) and Hakioawa, Kaho‘olawe (Figure 6). Stations at these sites have organic values that range from 7.4% to 23.6%.
Figure 2: Mean summary sediment composition statistics by island (%).
Figure 3: Frequency of occurrence of organic material for 91 stations (bin size=2%).
Figure 4: Sediment composition Island of Hawai’i (%).
Figure 5: Sediment composition Island of O‘ahu (%).
Figure 6: Sediment composition-Islands of Maui and Kaho‘olawe (%).
Statewide percentages of CaCO3 range from 1.5% to 96.5% (Figure 7). Ka‘apuna, Hawai‘i (Figure 4) is anomalous, having extremely low percentages of CaCO3 (<2%). Sediments from 31 of the 91 stations contain CaCO3 percentages greater than 90%. Sites on all MHI are represented in this group.
Figure 7: Frequency of occurrence of CaCO3 for 91 stations (bin range=5%).
Terrigenously based materials
Statewide percentages range from 1.4% to 99.7%. Only 11 of the 91 stations have greater than 50% terrigenous material. This is composed mainly of basalt or other land-based sediments. All Lāna‘i sites have much larger amounts of terrigenous material than Moloka‘i sites (Figure 8). Sites with greater than 75% terrigenous material are found on both the geologically oldest and youngest islands (Figure 4, 9)
Figure 8: Sediment composition-Islands of Moloka‘i and Lāna‘i (%).
Figure 9: Sediment composition-Islands of Kaua‘i and Ni‘ihau (%).
Principal components analysis using five variables including three grain-sizes, organic content and CaCO3, revealed three distinctive groupings from the 91 cases (Figure 10). In the first three axes, 93% of the variability is accounted for. Significant positive correlations were found between proportions of organic content and the silt/clay fraction (p=0.0) with a coefficient of determination of 67% (Figure 11). Likewise, there is also a significant relationship between CaCO3 and the largest grain-size, coarse and very coarse sand (p=0.01), and CaCO3 and the smallest grain-size, silt/clay (p=0.005).
The geologic age of the islands is significantly correlated with CaCO3 proportions (p=0.005), with older islands showing higher proportions of CaCO3 than younger islands.
Figure 10: Multivariate analysis of sediment composition and grain-size
Site abbreviations for RAT sites are as follows: 1st letter of island followed by 1st two consonants of site name, followed by transect number if more than one collection was made at a site.
Site abbreviations for CRAMP sites are as follows: 1st two letters of island followed by 1st three letters of site name followed by depth of site.
Figure 11: Regression analysis of proportions of organic matter and silt/clay fraction (<63 um) (p=0.001).
Sediment Composition and Grain-size
High organics and high silt/clay
Sediments containing high levels of organics and small grain-size (silt/clay) are indicative of areas heavily impacted by sedimentation and represent chronic disturbance to coral reefs. Sediment from all stations at both depths at Kāne’ohe Bay, O‘ahu, Hakioawa, Kaho’olawe, Pelekane Bay, Hawai‘i and the 3 m depth at Kakahai‘a, Moloka‘i contain a high percentage of organics (7% to 24%) and silt/clay material (9.2% to 63.1%). These 10 outliers are from four sites exhibiting the highest values among the 91 stations from 50 sites throughout the state. All of these sites with the largest proportion of organics and fine grain-size have been heavily impacted by chronic disturbances from sedimentation. Terrigenous organics are derived from land-based biotic material contributed through runoff. Kaho’olawe has a history of sediment loading due to devegetation from feral goats and bombing target practice. Sediment samples from Hakioawa, Kaho’olawe have a high percentage of fine grain particles and organic material, reflecting its past history of terrestrial sediment loading influenced by topography, vegetative cover and soil composition. Contributory anthropogenic stress factors such as sewage discharge and urbanization are not present at this site as they are at the other heavily sedimented sites throughout the state.
South Moloka‘i also has a long history of devegetation due to overgrazing (Roberts 2000). Pelekane and Kāne’ohe Bays have limited circulation allowing accumulation and resuspension of sediments. Kāne’ohe Bay has an extensive history of dredging and sewage discharge with considerable urbanization in the surrounding watershed. The main source of terrigenous materials into Pelekane Bay is through Luahine Gulch. Kawaihae harbor, adjacent to Pelekane Bay, has had extensive harbor development and modification that has interrupted long-shore sediment transport.
All stations on the islands of Kaua’i, Lāna‘i (except Nanahoa) and Ni‘ihau were found to be relatively similar in sediment associated organic content of sediments ranging from 2.7% to 4.0%. Nanahoa, with high organics (10.3%) receives extensive runoff from a denuded watershed during storm events.
High organics and low silt/clay
Sediments with high organics and low silt/clay may indicate anthropogenic stress from nutrification or enhanced fish feeding. Along with terrigenous input, organic contributions can be derived from marine sources such as decomposing algal material or fish detritus. Sites with high organic levels are highly correlated with the silt/clay fraction (<63 um) (Figure 11) with the exceptions of Hanauma Bay and Waikīkī, O‘ahu 4 m and Molokini, Maui. These anomalies have organic values close to 5%, ranking in the upper range of the majority of the stations, yet have very low levels of the silt/clay fraction typical of sedimented areas. The Waikīkī sediment was collected from a habitat composed mainly of macroalgae that is typical of a large portion of the inshore habitat in Waikīkī. The following are possible explanations for the high organics and low sitl/clay found at Hanauma Bay and Molokini.
High terrigeneous material
Three sites have a terrigenous sediment component that exceeds 80% (mainly basalt). Not surprisingly, Ka’apuna, Hawai’i, the most recent lava flow, has the highest percentage in the state: 3 m (99.7%) and 10 m (98.6%). Other sites with high percentages of basalt include Laupāhoehoe, Hawai’i and Lehua Island, Ni‘ihau. While sites on Hawai’i, the youngest island have a sediment composition high in basalt, sites on Kaua’i, the oldest of the MHI have very low levels. Most of the stations (87%) have organic values that range between 2% and 5%.
While 7 of the 50 sites had values higher than this range, only one site exhibited lower values at the other extreme of the spectrum. Ka‘apuna, Hawai‘i has very low organics and carbonate and high terrigenous material. The black basaltic sand found at this site is derived from a recent lava flow in the 1950’s.
Beaches at Waikīkī were artificially replenished with sands imported from Moloka‘i and Kahuku, O‘ahu. Thus, sediments from Waikīkī stations are all similar in composition and grain-size reflecting the contributing source. These sediments are high in CaCO3 and have a high proportion of coarse grains.
Of the four grain-sizes processed, only the largest grain-size is statistically different between depths. Although stations <5 m have an average of 8% silt/clay fraction, while those >5 m have considerably lower percentages of fines (5%), this was not found to be statistically different. This trend persists for fine and very fine sands (<5 m=15%, >5 m=14%). Medium sands at shallower depths (23%) are also similar to those at deeper depths (21%). The largest size fraction at depths <5 m (55%) and >5 m (36%) (p<0.001) is the only significantly different grain-size parameter.
In contrast, differences were found between depth categories in most sediment composition parameters. The average organic matter is significantly higher at depths below 5 m (6%) than at depths above 5 m (4%) (p=0.03), while CaCO3 shows an opposite trend. There is a significant difference between CaCO3 at shallower depths (67%) than at deeper depths (76%) (p=0.047). Average percentages of terrigenous material are relatively similar between depths, showing no statistically significant difference (<5 m=27%, >5 m=20%; p=0.08).
Most of the sites are relatively similar, consisting of high CaCO3 (>60%) and 2% and 4% organic material. Sites that deviate from this main group are clearly evident and can be predictive of the forcing functions driving the system.
Principle components analysis grouped samples, into three main clusters that deviate from the majority of the stations from sediment composition and grain-size (Figure 10). The cluster in the middle left of Figure 10, includes sites heavily impacted by sedimentation that are high in silt/clay and/or organics. The sites in this grouping include bays and sheltered sites. High silt/clay fractions are correlated with low coral cover. When fine sediments overwhelm the system, as they do at the sites within this group, sedimentation becomes the dominant forcing function on community structure.
The second cluster, at the bottom of Figure 10, includes sites that have high proportions of basalt and low levels of CaCO3 such as Ka‘apuna and Laupāhoehoe, Hawai‘i and Lehua Island, Ni‘ihau. Sites with these sediment characteristics have low coral cover and are primarily dominated by Pocillopora meandrina, a species found in shallow, high wave energy environments.
The third cluster, in the middle right of Figure 10, includes sites that are on exposed, north–facing shores, that are characterized by high proportions of large-grains and low proportions of fines. At sites like these exposed to high storm surf, sediments are reworked and fines winnowed by waves.
As expected, regressions show a carbonate latitudinal gradient across the islands. This stratification is statistically significant (p=0.005), showing a positive relationship between the age of the islands and CaCO3. Older islands have had a longer time for reef development and erosional processes to occur.
Last Update: 04/21/2008
By: Lea Hollingsworth
Hawai‘i Coral Reef Assessment & Monitoring Program
Hawai‘i Institute of Marine Biology
P.O. Box 1346
Kāne‘ohe, HI 96744