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Effects of Urbanization on Nearshore Ecosystems in Puget Sound: Liberty Bay Pilot Study
Puget Sound's rich ecosystem, which includes orcas, bald eagles, salmon, and hundreds of other species, directly or indirectly depends on the nearshore region for food, shelter, or reproduction. The processes that sustain the sound's nearshore ecosystem, however, are vulnerable to human activities, such as shoreline development, nutrient and contaminant inputs, boat traffic, and freshwater diversion or impoundment. As a result of urbanization, 33 percent of Puget Sound's shoreline has been altered, and more than 40 species of concern have significantly declined. Urbanization has many effects, such as altering the rate at which precipitation flows from the land into the sound, either as surface or ground water; it also affects water quality. Impervious surfaces, such as roads, roofs, and parking lots; septic systems; lawn fertilizers; and pesticides all contribute to these changes. Further changes are expected because the human population of the Puget Sound region is projected to increase by about 2 million within the next 15 years. To better understand how urban land use and development affect the nearshore environment, an interdisciplinary team of scientists from the U.S. Geological Survey's (USGS) Multi-Disciplinary Coastal Habitats in Puget Sound Project began exploring the effects of urbanization on the sound's nearshore processes and ecosystems. In 2006, they chose Liberty Bay, a semi-urbanized embayment adjacent to the city of Poulsbo, for a small-scale pilot study. This site was chosen because:
A preliminary look at the data from our 2006 study shows some results that raise further questions and suggest promising and exciting avenues for future research. Using public records, geographers Vivian Queija, Ray Watts, and Elisa Graffy compiled databases documenting spatial variation in urban attributes that alter natural watershed flows—for example, dwellings, septic systems, and impervious surfaces. They are developing methods to quantify additional attributes from images and other raw data. Early analysis shows that the density of land-ownership parcels along the shoreline correlates negatively with herring spawning; if there is a causal relationship, the specific mechanism is unknown. Untreated wastewater from leaking septic systems and sewers is an ongoing problem in Liberty Bay, resulting in elevated levels of coliform bacteria and shellfish-harvesting bans. In addition to coliform bacteria, wastewater typically carries excess nutrients, such as nitrogen and phosphorus, that can overstimulate the growth of algae, which, in turn, harms the natural ecosystem in various ways. Because of local wastewater inputs, we expected higher nutrient levels in Liberty Bay than at less developed Point Bolin, our reference site. In fact, water-column nutrient concentrations (measured weekly during April and May in discrete water samples collected 1 m below the surface and 1 m above the bottom) were generally higher at Point Bolin than in Liberty Bay. This pattern could reflect greater nutrient uptake in Liberty Bay, since warmer water temperatures measured within the bay may have increased the growth of phytoplankton and their consumption of nutrients, particularly nitrate and silica. To compare how primary productivity varied over time in Liberty Bay and at Point Bolin, the fluorescence of chlorophyll a (chl a)—a proxy for phytoplankton concentrations—was measured at both sites every 10 minutes 1 m below the surface during April and May. Measurements at both sites showed that three phytoplankton blooms occurred during these 2 months. Hydrologist Rick Dinicola collected water samples for nitrate isotopic analysis to help distinguish wastewater nitrogen from nitrogen derived from other sources. He also collected sediment samples for analysis of such wastewater indicators as pharmaceuticals and personal-care products (PPCPs, a broad array of substances, from prescription medicines to cosmetics and sunscreen). Future work will be done to determine where and how nutrients and PPCPs are delivered by shallow ground water discharging into Liberty Bay. To track the potential extent of wastewater influence on Liberty Bay's nearshore ecosystem, biologists Dennis Rondorf and Theresa Liedtke measured stable-nitrogen-isotopic ratios (δ15N values) in sediment and in organisms throughout the food web. These ratios provide information about the source of the nitrogen—for example, precipitation, fertilizers, and animal waste—and about trophic structures—the who-eats-whom relationships that define the food web. Wastewater input may lead to a stressed or shortened food web; δ15N values should indicate whether the trophic structures in Liberty Bay, where wastewater input is likely, differ from those at less developed Point Bolin. They might also show whether the marine plants in Liberty Bay use wastewater nitrogen. Mean δ15N values of two types of macroalgae were significantly higher in Liberty Bay than at Point Bolin, pointing to a possible wastewater influence. In spring 2007, sampling will target age- and size-specific organisms, and additional isotopic tracers will be explored to provide a clearer understanding of the trophic dynamics in Liberty Bay. To determine whether urban development has led to elevated metal concentrations, geochemist Renee Takesue measured total-metal concentrations in bottom sediment. Sedimentary metal concentrations were below the levels of concern set by the State of Washington; however, Takesue observed distinct spatial patterns, two of which are clearly associated with human activities. Chromium and nickel contents were elevated ubiquitously throughout Liberty Bay and at Point Bolin, probably as a result of historical inputs associated with activities at the Naval Undersea Warfare Engineering Station at Keyport. Lead and tin contents were elevated near a wrecked automobile abandoned on the beach. If urban runoff has contributed such metals as copper or zinc to Liberty Bay, the levels are low. To explore whether urban development affects benthic substrate and thus benthic communities, Takesue measured sediment-grain-size distributions in the study area. The grain-size data show bottom sediment to be significantly sandier and more consolidated adjacent to the city of Poulsbo than along undeveloped sections of shore. Development along the city's shoreline includes three municipal and private marinas, several docks, a waterfront street, and shoreline armoring; activities associated with building and maintaining these structures may affect the nearby bay floor. Geographers observed that shoreline armoring appears to have no correlation with the probability of herring spawning, possibly because herring spawn on subtidal vegetation or structures farther from shore. Armoring may have its greatest impact on the upper beach. Future work will explore the relationships between shoreline armoring, beach characteristics, and the success of beach-spawning forage fish, such as surf smelt and sand lance. The Liberty Bay pilot study is off to an excellent start: Team scientists have designed and implemented a fully interdisciplinary research and field plan, despite logistical obstacles. USGS scientists have established collaborative relationships with Washington Department of Fish and Wildlife biologist Dan Penttila, with Suquamish Tribe biologist Paul Dorn, and with local volunteers of the Liberty Bay Foundation (Luis Barrantes, coordinator). In 2007, team scientists will investigate further some of the intriguing results of the 2006 work and will commence the second phase of the pilot study, focusing on beach and nearshore processes, habitats, and ecosystem impacts associated with armored shorelines.
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in this issue:
Effects of Urbanization on Nearshore Ecosystems in Puget Sound Studying the Elwha River in Preparation for Dam Removal Sea-Floor Mapping Project Expands to South Shore and Cape Cod Bay
Google Maps View of Western Coastal and Marine Geology Projects Benthic Sponge Taxonomy Course at Mote Marine Laboratory
Nov. / Dec. 2006 Publications List |
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U. S. Department of the Interior | U.S. Geological Survey
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