Land Based Pollution
A High Concentration Of Coastal Development And Population Density Results In Cumulative Impacts to the Ecosystem
The majority of visitor accommodations, local human population, and infrastructure are along the coastline in Hawaiʻi. This high concentration of coastal development and population density results in most human activities taking place in and adjacent to the nearshore environment.
These are cumulative impacts that disrupt natural ecological processes and reduce ecosystem health, function, and resilience. Understanding the spatial distribution, intensity, and overlap of human activities and their cumulative impacts is essential for effective marine management and preserving the myriad ecosystem services generated by coral reefs.
The maps presented in this section are from a study and represent the first-ever assessment of spatial variation in cumulative impacts to nearshore ecosystems for the Main Hawaiian Islands.
This study generated a database of 16 individual human stressors and led a panel of experts to determine nearshore habitat vulnerability to each of these stressors. The three dominant nearshore marine habitat types in Hawai‘i are, in order of expert-assessed vulnerability (highest to lowest):
- coral reefs,
- rocky areas, and
- sandy or muddy areas.
Experts rated the vulnerability of habitats to each stressor based on four vulnerability criteria (frequency, trophic impact, percent change, and recovery time). Experts also determined their certainty for each rating. The cumulative impact score for any area (100 m grid cells) is based on the vulnerability weighted sum of the individual stressor intensities for the habitat type present in that area. Cumulative impact scores were normalized from 0-1 with values greater than or equal to the 99th percentile set to 1.
These maps represent a baseline of comprehensive human impact information for Hawai‘i and will be updated as new datasets and improved information on nearshore stressors become available. Please see Lecky (2016) for more detailed information on the maps and how they were generated. Certain map layers (e.g. fishing and sedimentation) are described in additional detail in Wedding & Lecky et al. (2018).
Agricultural and golf course runoff can introduce nutrients from fertilizers and chemicals such as pesticides and herbicides to coastal ecosystems.
A proxy was produced by calculating the area of agricultural land and golf courses by watershed. Agricultural land area was extracted from 2010 high resolution land use land cover data, golf course area was updated through 2016 via google searches and digitization from Google Earth and ESRI World Imagery Basemap. As with urban runoff, we aggregated the area by watershed to coastal source pixels and used a Gaussian decay function to estimate dispersal offshore, approaching zero at 2 km from shore. Map units are presented as a normalized score from 0 - 1.
Urban runoff can deliver a broad spectrum of land-based pollution that degrades nearshore water quality, with cascading effects on coral ecosystem health. Poor water quality can also undermine the natural defense abilities of corals and increase the likelihood of mortality from heat stress.
Pollutants can include household chemicals, oils, trash, sediments, and other toxicants. We derived proxy layers for the impact on nearshore marine ecosystems based on the total area of developed impervious surface (e.g. paved roads, parking lots, sidewalks, and roofs) in 2010, per watershed. We used USGS HU-12 watershed data to aggregate the impervious surface area by drainage to coastal source pixels and used a Gaussian decay function to estimate dispersal offshore, approaching zero at 2 km from shore. Map units are presented as a normalized score from 0 - 1.
Sediment from coastal erosion and various land-based activities can affect reef health by covering corals, blocking light, and inhibiting new coral settlement. This can lead to degradation of reef ecosystems. To quantify sedimentation, we modeled how much sediment was being transported into the nearshore marine environment around the Main Hawaiian Islands.
The annual amount of sediment (tons/yr) reaching the coast was calculated using the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) Sediment Delivery Ratio model for each of the eight main Hawaiian Islands. Sediment load is a function of land use and vegetation type, geology, soil characteristics, rainfall, slope, and hydrology. Only land areas that drain to a stream which reaches the coast and have a sediment supply were considered. The resulting sediment load at each point where a stream meets the coast was dispersed offshore using the Kernel Density tool in ArcGIS, resulting in a map of annual average sediment conditions offshore.
There are over 95,000 onsite sewage disposal systems (OSDS) (i.e., cesspools and septic tanks) used in Hawaiʻi, many close to coastlines and streams. These systems have varying levels of treatment capacity for nutrients, bacteria, and other pollutants found in wastewater and may leech into groundwater that flows to the ocean. Excess nutrients can promote rapid algal growth, outcompeting corals and disrupting the natural balance of the ecosystem.
Using findings from the Hawaiʻi Department of Health, this data consisted of estimated nitrogen flux and phosphorous flux from points representing Tax Map Key (TMK) parcels with OSDS in units of kg/day and effluent in gal/day (Whittier and El-Kadi 2009; 2014). We converted the points to raster by summing nutrient flux values within 100-m pixels and then calculated the total flux within a 1.5-km radius of each cell.
Fishing
Nearshore fisheries in the Main Hawaiian Islands encompass a diverse set of fisheries in which multiple gear types are used to harvest reef finfishes and invertebrates, estuarine species, and schooling coastal pelagic fishes. Communities in Hawaiʻi often rely on these fisheries for economic, social, and cultural services. Stress from over-fishing can cause ecosystem degradation and long-term economic loss.
We created a series of fisheries catch data layers for catch of reef finfishes, grouped into three categories of fishing platforms (non-commercial shore, non-commercial boat, and commercial) and three subcategories of fishing gear (line, net, and spear). For all fishing layers we accounted for marine protected areas (MPAs) where fishing is prohibited and de facto MPAs (e.g., military danger areas) where access is restricted. All fisheries catch layers represent average annual catch in units of kg ha-1 yr-1.
Non-commercial Shore-based Fisheries Catch – We used estimates of average annual catch by platform and gear type at the island scale, from 2004-2013, derived from Marine Recreational Information Program (MRIP) survey data. These island-scale estimates were combined with measures of shoreline accessibility (terrain steepness and presence of roads) to spatially distribute catch offshore around each island.
Line: Catch was extended 200 m offshore;
Net: Catch was extended offshore to the 20-ft (6.1-m) depth contour or a maximum distance of 1 km from shore;
Spear: Catch was extended offshore based on a decay function where catch decreases with depth to 40 m or a maximum distance of 2 km offshore, and assumes the vast majority of catch occurs shallower than 20 m.
Non-commercial Boat-based Fisheries Catch – We used estimates of average annual catch by platform and gear type at the island scale, from 2004‑2013, derived from Marine Recreational Information Program (MRIP) survey data. In order to spatially distribute catch offshore around each island, we used a function that decays with distance to boat harbors and launch ramps, and weighted the amount of catch out of each ramp/harbor based on the human population within the surrounding 30 km.
Commercial Fisheries Catch – We used average annual catch of reef fish by gear type over the years 2003-2013 as reported in commercial catch data collected by the State of Hawaiʻi Department of Aquatic Resources (DAR). Commercial catch is reported to DAR in large irregular reporting blocks, by gear and by species. Since it is not possible to distinguish between boat- and shore-based fishing activity with DAR’s gear categories, we assumed that catch is evenly distributed across each reporting block.
Aquarium fishing is reported to the State of Hawaiʻi Division of Aquatic Resources (DAR) separately from food fish, under a different licensing scheme and reporting system. Catch of aquarium species for the years 2003 - 2015 was mapped as the average annual number of individuals taken by reporting block (which differ slightly from commercial reporting blocks above). Seaward extent of reporting blocks was limited to 60 m depth and units were calculated to the number of individuals taken per hectare. No data was available to account for non-commercial take of aquarium species.
Direct Human Impact
Direct human impact includes trampling, swimming, disturbance or displacement of wildlife, and any other effects of human bodies physically in or near the water. To approximate this across the MHI, the InVEST recreation model was run statewide at 1 km resolution for the years 2005 – 2014. This model uses publicly visible geotagged photos posted to the photo-sharing website Flickr to calculate the annual average number of photo users per day per grid cell. A band of coastal pixels that intersect the shoreline was extracted from the model output and focal statistics with a 3 x 3 neighborhood was used to smooth the surface (accounting for inaccuracy in photo positions) and extend the data offshore. Map units are presented as a normalized score from 0 - 1.
Habitat Destruction
Dredging was defined as activity involving physically removing substrate with machinery typically to allow for safe passage of vessels. Polygons for dredged areas were extracted from NOAA habitat maps, NOAA/USACE maintained channels, and NOAA ENCs available as of 2016. Point locations associated with recent dredging projects in which the USACE consulted NOAA in regards to essential fish habitat were buffered by 100 m (Data from NOAA PIRO, unpublished). All features were assigned a value of 1 and converted to raster, resulting in a presence/absence layer of dredging.
Coastal engineering consisted of shoreline armoring structures (e.g., seawalls, revetments, groins, break waters), artificial land (i.e., land fill), and piers. Artificial shoreline, rip rap, and artificial structures were extracted from NOAA Environmental Sensitivity Index (2001) line data and NOAA habitat maps (2007) and validated with high resolution imagery. Note that the above datasets include Hawaiian fishpond walls as artificial man made shoreline structures. Resulting features were converted to raster and merged with all resulting pixels receiving a value of 1. To represent altered flow dynamics and offshore effects of coastal modification, focal statistics was used to create a 500 m buffer area with values ranging from 0.01 - 0.5 in proportion to how heavily modified the surrounding shoreline was.
Benthic structures are artificial features in the offshore environment that disrupt benthic habitat and include moored buoys, channel markers, offshore cables and pipelines. The primary source of input data was NOAA Electronic Navigational Charts (ENC) available as of 2016. Point features for buoys, beacons, piles, etc. were extracted from a total of 21 ENC layers and merged together. Line features for underwater cables from 3 ENC layers were merged together and hand edited for consistency across chart borders. Polygons for the Barbers Point oil pipelines (southwest O‘ahu) were extracted from NOAA habitat maps, and polygons for areas containing multiple cables, pipelines, or buoys were extracted from three NOAA ENC layers. All features were assigned a value relative to the estimated amount of seafloor they disrupt within a 100 m raster cell ranging from 0.001 (piles) to .1 (pipelines). All features were converted to raster, summing values when more than one benthic structure occurred in the same raster pixel.
Invasive Alien Species
Invasive algae can pose a serious threat to coral reefs by spreading and growing rapidly, smothering or outcompeting corals and other organisms. This can significantly alter the structure and function of the reef ecosystem. Four species of alien red algae have become invasive in Hawaiʻi: prickly seaweed (Acanthophora spicifera), hookweed (Hypnea musciformis), smothering seaweed (Kappaphycus spp.), and gorilla ogo (Gracilaria salicornia). Mangroves are an invasive alien species, introduced to Hawai‘i in the early 1900s, that rapidly colonize many nearshore environments, cause water quality issues, anoxic sediments, and provide habitat for invasive fish species.
A single data layer was created to represent presence-only of invasive algae species in nearshore waters and mangroves along the shoreline. Invasive algae data were from monitoring surveys in the Hawaiʻi Monitoring and Reporting Collaborative database (2000-2013) as well as invasive algae surveys conducted across the state in 2002 and 2004 by Dr. Jennifer Smith. Data consist of point locations of invasive algae presence that were subsequently converted to raster. To account for uncertainty in geographic position, the fragmentation and spread of algae, and expanded ranges, we estimated presence within a 1-km radius of observed invasive algae locations. Shorelines with invasive mangroves present were extracted from NOAA Environmental Sensitivity Index (2001) line data, converted to raster with a value of 1, and combined with invasive algae data. No buffering or focal statistics were applied with the assumption that mangroves do not extend further offshore than the footprint of the resulting 100 m raster pixels. These data are presence-only. The status in un-surveyed areas is unknown and there is the potential that a survey failed to observe an invasive species where it is actually present. Map units are presented as a normalized score from 0 - 1.
Ship Traffic
Data on ship locations for a 1-year period (August 2011 – August 2012) from satellite-based Automatic Identification System (AIS) was obtained from the Pacific Islands Ocean Observing System (pacioos.org). Point locations were converted to ship track lines by connecting points with a common ship identification number in chronological order. The line density tool in ArcGIS was used to calculate the density of ship tracks within 100 m raster pixels. The resulting ship traffic layer was used as a proxy for ship-based pollution. To represent the risk of ship groundings and wrecks the footprint of this ship traffic layer was clipped to 9 m depth. Map units are presented as a normalized score from 0 - 1.
Marine Debris
In 2015, high resolution (2 cm) aerial imagery of the coastlines within the MHI were obtained. This imagery was analyzed to identify and quantify densities of marine debris on all shores across the State. Point locations of marine debris items by type and size-class were obtained from the Hawai‘i Division of Aquatic Resources. These point data were converted to raster by summing the number of items, weighted by size class, within 100 m pixels. Then focal statistics was used to calculate the total amount of marine debris within 2 km of each coastal and marine map pixel, in order to extend the data offshore. This method assumes that the amount of debris washed up on a shoreline is indicative of the amount of debris on adjacent offshore reefs. Units of the final layer were normalized from 0 - 1.
Aquaculture
In addition to nutrient pollution from feed, other threats from aquaculture include risk of escape and genetic contamination of wild populations, disease, use of antibiotics and hormones, and attraction of animals. Two aquaculture map layers were developed for this project: ocean-based aquaculture and land-based aquaculture.
There have been two active Ocean-based aquaculture operations in the MHI. As of 2016, only one was in operation, offshore of Keahole Point on Hawai‘i Island, which grows Kahala (Seriola riviolana, marketed as Kona Kampachi) in an array of moored offshore net pens. The other operation, which grew Pacific Threadfin (Polydactylus sexfilis) off of Ewa Beach, O‘ahu, closed down in summer of 2012. It operated for 11 years, spanning much of the study period, and was therefore included in the dataset. For both operations, the footprint of the net pen array, dispersion fields for feed and feces, and “zone of mixing” designated by the DOH Clean Water Branch were digitized from environmental impact statements and monitoring reports. These were given arbitrarily decreasing values from 1 in the center to .1 at the edge of the zone of mixing.
Land-based aquaculture in the MHI consists primarily of shrimp ponds near the coastline, but also hatchery operations that stock offshore aquaculture and shrimp broodstock production. Footprints of terrestrial aquaculture operations were extracted from Hawai‘i State agricultural land use dataset (2015) and combined with footprints digitized using a list of aquaculture operations on the Hawai‘i Department of Agriculture website, Google searches, and identified using Google Earth and ESRI Imagery Basemap for visual validation. Aquaculture operation footprints were converted to raster with a value of one and decayed offshore using the same dispersion methods as the land based pollution layers.
Habitats
The same level of human-caused stress will have varying levels of impact on different habitat types due to differences in their vulnerability. For this study we used 3 broad categories of benthic habitat to map cumulative impacts. Nearshore reef habitats were mapped for all MHI by the NOAA National Center for Coastal and Ocean Science in 2007 using high resolution satellite (IKONOS) and aerial imagery. This was the primary data source for the three nearshore habitats mapped in this project. All non-land areas with known habitat properties were classified into one of the following three major habitat types: coral dominated hardbottom, other hardbottom, and softbottom. These classes were converted to raster at 100 m resolution with priority given to coral dominant hard bottom such that all cells with any portion covered by coral dominant hard bottom were classified as such, next priority was given to other types of hard bottom for the remaining cells, and finally soft bottom areas. Holes in the habitat maps, resulting from clouds, water clarity issues, white water, or missing imagery were filled using nearest neighbor nibbling and a 30 m depth contour line for the offshore extent.
Literature:
- Lecky J. Ecosystem Vulnerability and Mapping Cumulative Impacts on Hawaiian Reefs. University of Hawai‘i Mānoa, 2016, http://hdl.handle.net/10125/51453.
- Wedding LM & Lecky J, et al. Advancing the Integration of Spatial Data to Map Human and Natural Drivers on Coral Reefs. PLoS ONE, vol. 13, no. 3, 2018, doi:10.1371/journal.pone.0189792.