Report

Assessing the Condition of Hawaiian Coastal Wetlands Using a Multi-Scaled Approach

10/04/2010

Introduction to Coastal Wetlands and Condition Assessment
Coastal wetlands provide habitat for unique plant and animal species. They also protect offshore coral reef and sea grass ecosystems by cleansing polluted waters via nutrient recycling and filtering of sediments, metals, chemicals, and excess nutrients transported from agricultural and urban land-uses. Furthermore they help protect coastal properties from storm surges and floods. In short, both globally and locally, coastal wetlands provide a wide range of ecosystem services, many of which improve the quality of human welfare (Costanza et al. 1997).

Due to the vital ecosystem services provided by wetlands, it is important to have methods available for wetland condition assessment. Condition assessment can be used for a variety of purposes such as prioritizing sites for conservation, identifying sites for restoration, and monitoring trends in wetland condition at specific sites or multiple sites over time. It is also critical to determine whether restored and created wetlands are functioning like their natural counterparts. Thus, the overall question of this research is “What is the best method for assessing coastal wetland condition in Hawaii?” We propose that an integrated, multi-scaled approach using a combination of field inventories and GIS analysis is the most efficient and cost-effective way to assess the condition of Hawaiian coastal wetlands.

Tidal Fringe Marsh
Figure 1. Like many wetlands on O‘ahu, the Hamakua (tidal fringe) marsh is surrounded by urban development and is predominantly comprised of non-native plant species such as the invasive pickleweed (Batis maritima), shown here in the photo. Nevertheless, Hamakua marsh provides habitat for four endangered Hawaiian waterbirds.

Coastal Wetlands in Hawaii
Human alteration of coastal wetlands in Hawaii began around 800 AD when Polynesians first colonized the Hawaiian Islands (Vitousek et al. 2004). At the peak of the pre-contact Hawaiian era in approximately 1600 AD, almost all sizable saline and brackish water bodies were being utilized as fishponds (Kikuchi 1976). Furthermore, many of the freshwater wetlands, upslope from these fishponds, were used for taro (Colocasia esculenta) cultivation. Both the fishponds and taro fields were protected by cultural sanctions and the religious kapu (taboo) system. In fact, all bodies of water, from the smallest pool to the largest fishpond, were considered the home of guardian spirits, mo‘o, and contaminating them with sewage, corpses, etc. was absolutely prohibited (Kikuchi 1976). The expansion of the wetlands created by the Hawaiian irrigation system allowed the permanent colonization by waterbirds, and favored some native species of plants and animals at the detriment of others.    

The arrival of Europeans in 1778 brought about drastic changes for the Hawaiian culture and by 1930 most of the Hawaiian fishponds and taro fields had been abandoned (Kikuchi 1976). Hawaiian wetlands were filled and drained to make way for the expansion of agriculture, such as sugar cane and pineapple plantations. Deforestation in upland areas increased sedimentation into waterways and many coastal wetlands were transformed into harbors, military bases, or urban developments. Introduced plants and animals exacerbated the degradation of wetland ecosystems and by 1940, Hawaii’s six endemic waterbird species were classified as endangered (U.S. Fish and Wildlife 2005). Since then, increasing development for airports, residential subdivisions, and resorts has continued to directly and indirectly threaten coastal wetland ecosystems (Figure 1). By 1980, at least thirty percent of the natural lowland wetlands had been lost, with the remainder seriously degraded from altered hydrology and the invasion of non-native species (Erickson 2006).

Endangered Waterbirds
Figure 2. Photo of two endgangered Hawaiian waterbirds: the Black-necked stilt (Himantopus mexicanus, left) and Hawaiian coot (Fulica alai, right).  Photo by National Park Service

Wetland Condition Assessments
In order to comprehensively assess the functional integrity of wetlands, the U.S. Environmental Protection Agency (EPA) advises using a multi-scaled approach that includes intensive field surveys, rapid on-site assessments and remote or landscape-scale assessments using a Geographic Information System (GIS). While intensive field surveys are valuable and provide detailed analysis of habitat condition, they require tremendous effort and are costly to conduct. Rapid and remote assessment procedures, on the other hand, are more expedient and are correlated with vegetation field surveys (Mack 2006) and water quality data (Brown and Vivas 2005). Unlike intensive field surveys, rapid assessment methods use field indicators to assess the condition of wetlands relative to baseline or unaltered conditions. For example, habitat heterogeneity and proximity to other wetlands are indicators of wildlife utilization.

The ultimate goal of condition assessments is to collect sufficient data to determine whether or not a wetland is meeting performance goals and how the functional integrity of a wetland may be changing over time. Initially data from more detailed assessments can be used to calibrate rapid and remote condition assessments. Once remote and rapid assessments methods have been tested and proven to reliably ascertain the health of wetlands, they may in some cases be used in lieu of more detailed and expensive field surveys.

A multi-scaled approach, using a combination of field inventories and GIS analysis to assess the condition of wetlands, is still in the early stages of development in Hawaii. In 2007, survey crews began collecting detailed field data for forty wetlands along the leeward and windward coastlines of the five main Hawaiian Islands. These sites included created, restored, and natural wetlands in isolated and developed landscapes in various salinity classes (freshwater, brackish, euhaline, hyperhaline; see definitions in Table 1).

Table 1

Results revealed that the quality of the soils tended to be higher in natural wetlands compared to created and restored wetlands (Bantilan-Smith et al. 2009) and water quality was negatively correlated with the percentage of development within a 1000 m radius (Bruland and MacKenzie 2010). The composition of plant communities of all wetlands in the study was predominantly non-native; only 16 of the 85 plants were identified as native species (Bantilan-Smith et al. 2009).

Table 2

Preliminary Condition Assessment Results for Hawaiian Wetlands
A new project is underway to determine whether or not remote and rapid assessment methods can detect a gradient in the condition of Hawaiian landscapes and coastal wetlands. Using a GIS, landscape development indices (LDI) were calculated for the watersheds of twenty wetlands. The LDIs provide a quantitative measure of human development on a scale from 1 (for natural landscapes) to 10 (for central business districts).

Preliminary analysis has indicated a strong relationship between landscape development intensity indices of watersheds and the quality of the soils and water in Hawaiian coastal wetlands (Figure 3). Among twenty wetlands, surface water total dissolved nitrogen (TDN) was most strongly related to watershed conditions (r2 = 46.4, p < 0.001). Soil bulk density was also strongly related to the intensity of development within the watershed (r2 = 35.3, p = 0.008), perhaps because development increases the transport and deposition of mineral soils or simply because wetlands located in an urbanized watershed are more likely to have compacted soils due to more frequent human use.

Figure 3
Figure 3. Relationship between watershed LDI values and the total dissolved nitrogen in the surface water (top) and bulk density of soils (bottom). High levels of total dissolved nitrogen and soil bulk density were measured at both the Pouhala (red) and Wahiawa (blue) marshes.

In some cases, however, development in close proximity to wetlands can overwhelm the impacts occurring at the watershed scale. For example, on the island of O’ahu in the Pouhala and Wahiawa marshes, both of which are located along the Pearl Harbor coastline but in different watersheds, the LDI values are very different for each wetland. The watershed of the Pouhala marsh is almost completely developed and therefore received the highest LDI value (6.9) of all the watersheds in this study. The upper portion of the much larger watershed that drains into the Waiawa marsh is undeveloped, however, which lowered the LDI score calculated for that watershed to 2.7 (Figure 3). Nonetheless, some of the highest TDN and soil bulk density values were measured at both of these marshes, more than likely due to the intense levels of development and harbor activities surrounding these wetlands.

Classified as tidal fringe wetlands, the Pouhala and Waiawa marshes both receive reduced tidal exchange between fresh and marine water sources due to ditches and berms in the immediate vicinity of the marshes. For example, the Kapakahi stream associated with the Pouhala Marsh is channelized, lacks natural vegetated buffers and is commonly filled with garbage. These features resulted in low rapid assessment scores in categories such as water source, hydroperiod (natural fluctuation of water) and, hydrologic connectivity. Thus fully restoring the functional integrity of the Pouhala and Waiawa marshes will require managers to mitigate impacts from local land uses that are impairing the water quality of these wetlands.

These results suggest that immediately adjacent land use may have a larger impact on wetland status than the overall watershed characteristics and therefore understanding stressors associated with adjacent land uses and managing for them will be important for the success of future wetland restoration in Hawai‘i.

" Ideally, a healthy, restored wetland is self-sustaining and can function independently without human intervention."

In the next phase of the project three rapid field assessment methods will be used to assess the condition of 27 wetlands (previously analyzed), on the islands of Oahu, Maui, and Hawaii. The study sites will include created, restored, and semi-natural wetlands in various salinity classes (freshwater, brackish, euhaline, hyperhaline) as well as recently abandoned taro fields and fishponds. Furthermore, the wetlands represent a gradient of disturbance, within different hydrogeomorphic classes and ecoregions. The dominant presence of invasive species suggests that it will be difficult to locate reference wetlands that are representative of culturally unaltered or “best attainable” conditions. This, however, is not entirely unusual since reference wetlands in nearly pristine condition exist in only a few ecoregions, and even the concept of a reference ecosystem is highly debated.


In order to determine which method works best for the Hawaiian coastal wetlands, three rapid condition assessment methods will be tested: Wetland Rapid Assessment Procedures ([WRAP] Miller and Gunsalus 1997), California Rapid Assessment Methods ([CRAM] Collins et al. 2008), and the draft Hawaii Method (based on the Hydrogeomorphic [HGM] Approach, SAIC 2004). Of the three methods, WRAP is the most rapid and least quantitative, whereas the CRAM methods provide a more objective assessment and are the most thoroughly documented. The Hawaii method, on the other hand, the most labor intensive and detailed, and separate score cards for each functional category. For example, the functional categories for tidal wetlands are: 1) dissipation of energy, 2) retention of imported elements, 3) interspersion and connectivity, 4) characteristic plant community, 5) characteristic invertebrate food webs, 6) characteristic vertebrate habitats, and 7) habitat for threatened and endangered species.

The individual assessment scores can be combined or used separately to prioritize wetlands based on their functional values and specific restoration goals. For example, wetlands that are invaded by mangroves (Rhizophora mangle) lack a “characteristic plant community” but still function to protect shorelines and offshore coral reef habitats. If the main goal, however, is to restore habitat for threatened and endangered species, wetlands that have suitable or “characteristic” habitats would rank higher on the priority list. Healthy wetlands that provide multiple functions receive the highest overall scores.

Landscape Map
Figure 3. Map of landscape development intensity within the watersheds of the Pouhala and Waiawa marshes, located along the coastline of Pearl Harbor, O‘ahu. The watersheds of the Pouhala marsh (west) and Waiawa marshes are outlined in yellow.

 

Benefits of a Multi-Scaled Approach
This type of multi-scaled approach to assessing and monitoring the condition of wetlands can facilitate efforts to restore the functional integrity of coastal wetlands. In the process of conducting a condition assessment an observer takes into account the wetland’s position in the landscape and its connectivity or lack thereof to adjoining ecosystems, which can either constrain or support restoration efforts. Furthermore, the suite of metrics can be viewed as a set of ecological performance standards for wetland mitigation and restoration practitioners. In addition to providing guidelines for ranking wetlands and setting achievable restoration goals, they can serve as a way to measure the long term success of mitigation efforts.

The desire to provide specific hydrological conditions (water quality and quantity) makes wetlands difficult to restore. Nonetheless a wetland manager can improve their chances of success if they have access to information from detailed field surveys, as well as from remote and rapid field assessments when developing a restoration plan. The information collected at all scales should inform restoration managers; the detailed soil and water quality data as well as the more remote landscape-scale assessments can be used to assess and monitor the condition of wetlands over time.

Ideally, a healthy, restored wetland is self-sustaining and can function independently without human intervention. However, due to the long history and geographic scope of human disturbance along Hawaiian coastlines, steady state conditions are not realistically achievable for lowland coastal wetlands given the prevalence of invasive species, continued development pressures as well as sea-level rise. Instead, these ecosystems will require continued active and adaptive management if they are to persist into the future. Rapid and remote assessment procedures can help reduce the cost of monitoring wetland conditions and thus facilitate an adaptive management system that can rely on easily identifiable indicators of ecosystem health.

 


References

 

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Keywords:

Wetland, Condition Assessment, Watershed, Landscape, Functional Integrity

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