Algal bloom at the Boquillas site credit: John Villinski

Brooks (UA-HWR)

Research objectives for this project, begun in March 2001, have been refined and reprioritized. The qualitative and quantitative differences between mesic and arid ecosystems means that there are more baseline measurements that need to be performed before some of our original research questions can be answered. Consequently, we have taken two approaches to address these broader issues of hillslope, riparian, and stream connections in arid and semi-arid environments. First, we have coordinated our research with other SAHRA investigators within TA3 as well as across other thrust areas. Secondly, we have leveraged our SAHRA funding to obtain supplemental resources to make specific measurements and fund analyses.

Science Questions

• How are DOC, DON, and particulate C and N delivery to the aquatic systems related to changes in flow regime?



• What are the major controls on carbon and nitrogen concentrations in the Rio Grande drainage? (collaborative with Hogan and Villinsk)



• How are the concentration, structure, and composition of inorganic and organic carbon and nitrogen related to flowpaths and hyporheic processes in the San Pedro? - (collaborative with Conklin)

Episodic flow events transport large amounts of terrestrial organic matter to the aquatic environments in semi-arid streams. Early during very large flow events, particulate organic matter (POM) exceeds dissolved organic matter (DOM) by a factor a two. During moderate flow events and between storms POM declines rapidly, presumably due to settling, while DOM remains elevated. The first few storms of the monsoon season result in a tripling of dissolved organic carbon (DOC) concentrations in the surface waters of the San Pedro River. Concentrations during subsequent storms are intermediary between baseflow values and the initial increase in concentrations. Baseflow concentrations exhibit very little variability during the remainder of the year. Dissolved organic nitrogen (DON) concentrations increase by a factor of 10 with the first few storms of the monsoon season, and then decline sharply to pre-monsoon values. Unlike DOC concentrations that remain relatively stable during the winter, DON concentrations increase again late in the year and remain elevated until spring. With the exception of the first flow events, the ratio of carbon to nitrogen (C:N) in DOM increases during the monsoon. Winter C:N ratios remain close to 10:1, similar to living biomass.

Chemical analyses of organic matter sources throughout the year confirm that the increases in DOC and DON during the monsoon are the result of terrestrial organic matter being transported to surface water (Figure 3-6). As the monsoon ends, dissolved organic matter gradually becomes dominated by aquatic sources, consistent with a late summer increase in in-stream primary production and absence of a transport vector for terrestrial organic matter. In-stream autochthonous production continues to dominate the available organic matter throughout the winter.

Current understanding of the connections between riparian and upland ecosystems is based on temperate, mesic ecosystems that are qualitatively and quantitatively different than semi-arid environments. This study has provided baseline information that can be used to develop external, integrative, interdisciplinary proposals that address the key questions regarding the importance of hillslope land use change to the function and sustainability of these ecosystems. Specifically, this work has demonstrated the importance of episodic hydrologic transport events in the overall carbon and nitrogen balance of semi-arid river systems. These findings are analogous to findings from other seasonally flushed catchments, e.g. snowmelt dominated ecosystems. Because most arid and semi-arid streams ultimately originate in seasonally snow-covered high elevation environments, the emerging conceptual basis for understanding the structure and function of these systems invokes a pulsed ecosystem concept. The pulsed availability of water occurs on a variety of time scales from headwater reaches to main-stem rivers. These periods of increased water availability are associated with pulses of biological activity. Because they also transport the limiting nutrients and carbon required for biological processes, they appear necessary for the biogeochemical processes at the base of ecosystem function. Further quantification of the linkages between upland and aquatic environments is key to one of the goals of SAHRA: identifying the baseline sustainability of these riparian ecosystems.

Plans

The majority of hydrochemical and biogeochemical research occurring on the San Pedro River is occurring at a few intensively studies locations. Results from this work are yielding important, mechanistic insights into the structure and function of riparian ecosystem in arid and semi-arid environments. Because many of the insights gained from this research are new, there are few (if any) data sets that can be used to expand the results from these areas to other reaches along the greater San Pedro. Results from this and other SAHRA projects suggest that future research directed toward understanding the structure, function, and sustainability of aquatic ecosystems in arid and semi-arid regions involves expanding the spatial and temporal insights gained over the last few years to larger spatial domains. To that end, we propose to focus our work on the following two general areas:

Question A. Do the intensively studied sites at Boquillas and Greyhawk on the San Pedro River represent water chemistry and stream riparian interactions throughout the larger San Pedro system?
A1. Will sites similar to the intensively studied reaches on the San Pedro exhibit similar patterns in water quality driven by hydrology?
A2. Are water quality and biogeochemical cycles along any specific reach more closely related to instream/upstream processes or to the terrestrial environment?

Question B. Are the results from the San Pedro applicable to other arid and semi-arid river systems?
B1. Are the sources of DOM in the Rio Grande similar to those identified for the San Pedro?
B2. How are riparian environments linked biogeochemically to adjacent hillslope and terrace environments?
B3. Do the large amounts of available nitrogen in semi-arid regions promote nitrate leaching to groundwater?

As discussed above, the majority of the biogeochemical research within SAHRA has occurred along the San Pedro River. There is general consensus that over the next two years this research needs to be expanded to include other semi-arid environments as well as integrated with other research projects within SAHRA. These three questions are designed to begin to address this need. Specifically, question B1 expands biogeochemical research out of the San Pedro and into the Rio Grande where there is significant research by other thrust areas. Question B2 is designed to link terrace research with riparian research building on the wealth of information in the San Pedro. Question B3 conceptually links biogeochemical research with questions of recharge.