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The carbon cycle of the coastal ocean is a dynamic component of the global carbon budget. But the diverse sources and sinks of carbon and their complex interactions in these waters remain poorly understood. Here we discuss the sources, exchanges and fates of carbon in the coastal ocean and how anthropogenic activities have altered the carbon cycle. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide during post-industrial times. Continued human pressures in coastal zones will probably have an important impact on the future evolution of the coastal ocean's carbon budget.

Shifts in bacterioplankton community composition along the salinity gradient of the Parker River estuary and Plum Island Sound, in northeastern Massachusetts, were related to residence time and bacterial community doubling time in spring, summer, and fall seasons. Bacterial community composition was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. Average community doubling time was calculated from bacterial production ([ 14 C]leucine incorporation) and bacterial abundance (direct counts). Freshwater and marine populations advected into the estuary represented a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community formed at intermediate salinities in summer and fall, when average doubling time was much shorter than water residence time, but not in spring, when doubling time was similar to residence time. Sequencing of DNA in DGGE bands demonstrated that most bands represented single phylotypes and that matching bands from different samples represented identical phylotypes. Most river and coastal ocean bacterioplankton were members of common freshwater and marine phylogenetic clusters within the phyla Proteobacteria, Bacteroidetes, and Actinobacteria. Estuarine bacterioplankton also belonged to these phyla but were related to clones and isolates from several different environments, including marine water columns, freshwater sediments, and soil.Estuarine waters contain strong biological and chemical gradients established by the mixing of freshwater and seawater and modified by autochthonous biological activity. Many of these gradients, including salinity, nutrient concentration, organic matter composition, and bacteriovore community composition, are thought to influence the composition of natural bacterioplankton communities (2, 11). Such changes in environmental conditions, when recreated in mesocosm and microcosm experiments, caused shifts in the phylogenetic composition of bacterioplankton communities (10,19,32,41). It is therefore reasonable to predict that similar shifts will occur in natural freshwater and marine bacterioplankton communities when they encounter estuarine gradients, leading to the development of an estuarine community.Several studies have described estuarine microbial diversity and some have demonstrated how freshwater and marine bacterioplankton communities mix along estuarine gradients (3,4,8,14,21,37), but few reports have provided evidence of a unique estuarine bacterioplankton community. This is partly due to the dynamic nature of estuaries and the difficulty in distinguishing estuarine populations from those that wash in from adjacent environments. Crump et al. (6) identified putative estuarine bacteria associated with particles in the Columbia River estuarine turbidity maximum (ETM) by comparing environmental clone libraries of PCR-amplified 16S ribosomal DNA (rDNA) from the river, the estuary, and the coastal ocean. Similarly, Hollibaugh et al. (14) demonstrated the mixing of bacter...

We present a conceptual approach for evaluating the biological and hydrological controls of nutrient removal in different sized rivers within an entire river network. We emphasize a per unit area biological parameter, the nutrient uptake velocity (νf), which is mathematically independent of river size in benthic dominated systems. Standardization of biological parameters from previous river network models to νf reveals the nature of river size dependant biological activity in these models. We explore how geomorphic, hydraulic, and biological factors control the distribution of nutrient removal in an idealized river network, finding that larger rivers within a basin potentially exert considerable influence over nutrient exports.

w ww ww w. .f fr ro on nt ti ie er rs si in ne ec co ol lo og gy y. .o or rg g © © The Ecological Society of America B eyond climate, land use -and its manifestation as land-cover change and pollution loading -is the major factor altering the structure, function, and dynamics of Earth's terrestrial and aquatic ecosystems. Urbanization, in particular, fundamentally alters both biotic and abiotic ecosystem properties within, surrounding, and even at great distances from urban areas (Grimm et al. 2008). Around the world, rates of land change will increase greatly over the next 20-50 years, as human populations continue to grow and migrate (Alig et al. 2004;Theobald 2005). The nature, pattern, pace, and ecological and societal consequences of land change will vary on all spatial scales as a result of spatial variation in human preferences, economic and political pressures, and environmental sensitivities (Carpenter et al. 2007). To respond, we must determine how variables influence land change and ecosystem properties at multiple interacting scales, and understand feedbacks to human behavior.Human social and economic activities drive land change at all scales, and may enhance or hinder the movement of materials via wind, water, and biological and social vectors, sometimes in surprising ways that cut across scales (Kareiva et al. 2007;Peters et al. [2008] in this issue). For example, individual human decisions can influence regional dynamics within a continent when many people respond similarly to the same economic or climatic driver; the Dust Bowl in the North American prairies during the 1930s is a historical example of such cumulative effects (Peters et al. 2004). Individual decisions can also influence broad-scale land-change dynamics on other continents; for example, a switch to soybean production in South America is being driven by market demand from China. In turn, the changes wrought by humans produce ecosystem dynamics that feed back to influence resource availability and human well-being. Human responses may ameliorate or exacerbate these effects. Thus, there are complex interactions and feedbacks between the direct manifestations of human activ- Urbanization, an important driver of climate change and pollution, alters both biotic and abiotic ecosystem properties within, surrounding, and even at great distances from urban areas. As a result, research challenges and environmental problems must be tackled at local, regional, and global scales. Ecosystem responses to land change are complex and interacting, occurring on all spatial and temporal scales as a consequence of connectivity of resources, energy, and information among social, physical, and biological systems. We propose six hypotheses about local to continental effects of urbanization and pollution, and an operational research approach to test them. This approach focuses on analysis of "megapolitan" areas that have emerged across North America, but also includes diverse wildland-to-urban gradients and spatially continuous coverage of land change. Conc...