Recognized as one of the most productive ecosystems in the world, tidal salt marshes are sinks for oxygen and inorganic nutrients while sources of organic carbon (Arrigoni et al., 2008; Childers et al., 2000). The areal extent of salt marshes is small in proportion of total global land-cover but they are still considered potentially significant as a source of dissolved organic carbon to the ocean (Frost et al., 2006). It is also suggested that at mid-high latitudes, the continental shelves are net sink for CO2 and near shore and estuarine waters are net source (Cai et al., 2006).

The importance of salt marsh-dominated estuaries is recognized by scientists and policy makers under the need to understand the carbon cycle, and the ability of marshes to provide important ecosystem services such as nutrient abatement. The marine-dominated estuary is differentiated from river-dominated estuaries by often receiving little freshwater besides direct precipitation and groundwater, and therefore, the material exchange between marine-dominated estuary and salt marsh is mainly driven by the tidal cycle (Jiang et al., 2008).

To find out the function of salt marshes in marine-dominated estuaries, it is very important to look into the detailed pattern of materials transport qualitatively and quantitatively in a relatively high spatial, temporal, and material resolution. Figure 1 illustrates the function of salt marsh as the carbon pump to estuarial water, and because these carbon will not be released to the atmosphere, it is an important source for the continental shelf carbon sink (Jahnke, 2008). Traditional methodology such as collecting samples by boat can assess the concentration change of materials between flooding tide and ebb tide to understand the qualitative role (source or sink) of salt marshes to the carbon cycle. But the low temporal resolution of these measurements, the very high variability from one tidal cycle to the next cycle, and the difficulties in calculating water flux makes it difficult test the mechanisms behind carbon exchange using this approach.

Figure 1. Plantation in salt marsh absorbs CO2 from atmosphere by photosynthesis and releases carbon in different form to the ocean by tidal water (Jahnke, 2008).

The development of technology allows the researchers to have more tools to raise the temporal resolution of their measurements. One example is the emergence of Infra-red (IR) analyzers and other spectrophotometry applications. These highly accurate applications make it possible to make field deployable long-period measurement for CO2, colored dissolved organic matter (CDOM), Chlorophyll-a, and suspended solids. IR analyzers are widely used in aquatic system research combined with "shower-head equilibrators" to measure CO2 concentration in the water body, and CDOM and suspended solids is also a proxy probe for dissolved organic carbon (DOC) and particulate organic carbon (POC).

In the summer of 2007, using funds from the Connecticut Sea Grant, we have conducted a high frequency monitoring field study measuring CO₂,Chlorophyll-a, and oxygen. Together with an Acoustic Doppler Current Profiler, we were able to make high frequency measurements of the flux of these materials in a small salt marsh dominated estuary of Long Island Sound. And from the result we are able to identify several qualitative characteristics of this site.

Figure 2.The qualitative model of net flux.

1. The salt marsh consume oxygen and chlorophyll-a, and exports CO2 to the estuary, indicating the salt marsh is a oxygen sink and CO2 source, which reflects respiration and decomposition process is stronger than photosynthesis when the tide is flooding the salt marsh.
2. The concentration of oxygen and chlorophyll-a has a positive relationship, but the R square is 0.38, which indicates the phytoplankton photosynthesis contributes the oxygen concentration in the water but there are other factors such as plants in the marsh contributing to the change.
3. The more daylight hours that a tidal cycle stays in, the more chlorophyll-a and oxygen that can be produced in the salt marsh, and the less net CO2 that can be exported to the estuary. During periods of more sustained marsh flooding the sink term for O2 also increases, potentially due to either a decrease in primary production (which is corroborated by the Chl-a data) or an increase in respiration. CO2 doesn't have a relationship with flooding duration because of the buffer of bicarbonate and carbonate (Figure 2).