In Corn Cob Canyon Creek, the median SRP concentration was 0.11 mg/L at Lewis Road, where the stream emerges from an underground culvert, and 2.2 mg/L at Hudson Landing, downstream of row crops. At the downstream locations in both Carneros and Corn Cob Canyon Creeks, SRP concentrations exceeded the 0.12 mg/L target level in 100% of biweekly samples. In Uvas and Llagas Creeks in the south Santa Clara Valley, SRP concentrations were generally below the target SRP concentration of 0.12 mg/L at all locations. However, water quality problems occurred more frequently downstream of agricultural land use , where a greater percentage of collected samples were over the target concentration . In the Pajaro River, elevated SRP concentrations occurred in the river’s upstream reaches at Chittenden Gap, due in large part to flow from San Juan Creek and associated ditches that drain irrigated fields in the San Juan Valley. In contrast to tributaries draining the south Santa Clara Valley, San Juan Creek had elevated SRP levels , and was a particularly significant source of nutrients to the Pajaro River during summer months, when flow from other creeks declined. In addition to San Juan Creek, several agricultural ditches in the south Santa Clara and San Juan Valley regions that flow intermittently may also contribute nutrients to the Pajaro River. We hope to address these issues in future research.In addition to agriculture,stackable planters natural processes and urban runoff may also contribute phosphorus to waterways.
Although no increase in phosphorus levels was detected at urban sampling locations on Llagas and Uvas Creeks , urban runoff from the city of Watsonville may contribute to elevated SRP levels in Watsonville Slough and the lower Pajaro River, particularly during the winter storm season. At Ohlone Road, our most upstream site in the slough, surface runoff from the city of Watsonville may also contribute to elevated SRP levels during winter storms. It is also worth noting that Watsonville Slough at Ohlone Road had a period of very high SRP concentrations in the late summer through fall of 2003; these may be associated with erosion from a development project that occurred adjacent to the sampling site.Algal growth is generally limited by available nutrients. In freshwater systems, an increase in either phosphorus and/or nitrogen can stimulate production . Agronomically small losses from the farm are sufficient to stimulate algae growth in lakes and streams. For example, 0.03–0.05 mg PO4 -P L-1, which are very low concentrations, stimulate high growth rates in algae. The excess growth of algae or aquatic plants, a process termed eutrophication, can threaten drinking water supplies by creating toxic conditions, fouling water intakes, and changing the availability of oxygen in the water. In addition to compromising drinking water quality, elevated nutrient levels may increase or decrease the abundance of specific species in a freshwater system. The change in species abundance can affect the taste and odor of water, making it unpalatable or even toxic to some organisms.
These potential ecosystem changes have not been investigated in Central Coast surface waters. With the increase in algae, levels of dissolved oxygen in the water column during the day can become very high . At night the activity of microbes that break down decaying organic matter in the sediments can reduce oxygen concentrations to the point that some aquatic species have difficulty surviving. Thus, very high, low, or fluctuating concentrations of dissolved oxygen can indicate eutrophic conditions. Although nutrient levels influence the growth of algae, the overriding factors that control algae growth in streams are disturbance , light availability , and consumption by animals. Thus, even if nutrient levels are elevated, excess algae growth may not occur. This fact severely complicates the development of an enforceable numeric standard for phosphorus along the Central Coast, as it is difficult to find a direct relationship between nutrient levels and algal growth.We detected seasonal changes in SRP concentrations in many waterways. One prominent seasonal pattern was an increase in SRP concentrations during the late summer in waterways that receive discharge from cultivated lands. This late summer increase occurred in San Juan Creek, in the Pajaro River at Chittenden Gap, and in Corn Cob Canyon Creek , and may be due to the combined effects of irrigation discharges and decreasing stream flows, which limit the capacity of waterways to dilute nutrient inputs. In contrast, Watsonville Slough had its highest SRP concentrations from fall through spring, with concentrations declining to an annual low point in mid summer .
High SRP concentrations in the winter rainy season may be associated with increased surface runoff from agricultural fields located along the slough. Tile drains may also facilitate subsurface loses of phosphorus . In Carneros Creek, which is dry from approximately May until December each year, a third seasonal pattern emerged . SRP concentrations were moderately elevated at both upstream and downstream sites following the first winter rains, which suggests that soil phosphorus accumulates over the summer months and is flushed into the creek with the first rains. At Dunbarton Road where there is little cultivation upstream, sources may include natural decomposition in grasslands, cattle grazing, and rural residential land use; at downstream sites sources also include agricultural land use. At San Miguel Canyon Road, the downstream location, SRP concentrations increased again in the late winter and spring of 2002 and 2003, reaching very high levels that frequently exceeded 1 mg/L. Nutrient concentrations were highly erratic in 2002 and 2003, and subsequently declined in 2004, suggesting that nutrients originated from a point-source that has ceased to discharge. No seasonal concentration trends were observed in the upstream tributaries of the Pajaro River . At these locations SRP concentrations remained low throughout the year. We calculated the SRP load discharged by each tributary , and found loads varied seasonally corresponding with discharge . The SRP load was greatest at Chittenden during January and February, when discharge was also greatest. San Juan Creek was not sampled during this period, but likely accounts for a significant portion of the unaccounted load because it has elevated SRP concentrations and yearround flow. In the Pajaro River, there is a strong seasonal trend in SRP concentrations . Concentrations decline after the rainy season ends. Because SRP concentrations remain relatively high in the winter, rainfall probably transports SRP to surface waters. Furthermore, the loss of SRP from Santa Clara/San Benito Counties is highest during these rainfall periods . Because concentrations and export of SRP in the Pajaro River are rainfall dependent, it is difficult to determine long-term trends independent of recent rainfall patterns.Elevated phosphorus concentrations can cause excessive algal growth in waterways, and preventing excessive growth is the primary reason phosphorus concentrations are regulated. Algal biomass in the water column can be determined from the concentration of chlorophyll a,stacking pots which indicates the degree of excessive algal growth. We monitored chlorophyll a concentrations at several sites in the Pajaro River watershed on a biweekly basis and compared these concentrations to phosphorus. We found no direct relationship between chlorophyll a and phosphorus levels at any of the locations . The lack of a direct correlation between chlorophyll a and phosphorus levels indicates that P availability is only one of the factors controlling algal growth. Canopy cover and turbidity , algae-eating organisms , substrates that allow different types of algae to attach, and algae sources also play a role in algal growth and chlorophyll a concentrations. Furthermore, additions of nitrogen can stimulate algal growth in streams and rivers, which challenges the commonly held belief that phosphorus is the nutrient that controls the growth of algae in freshwater ecosystems. Our research group from the Center for Agroecology and Sustainable Food Systems has begun efforts to assess the growth patterns of algae in order to determine how elevated phosphorus and nitrogen levels influence these patterns. Under state legislation known as the Agricultural Discharge Waiver that took effect in January 2005, farmers are required to develop farm water quality plans to protect surface waters along the Central Coast. One goal of our research is to inform growers of current water quality conditions in waterways adjacent to their land so that they can take steps to reduce their impacts on waterways while continuing to farm profitably.
Because phosphorus is transported to waterways in storm and irrigation runoff, reducing soil erosion and surface runoff is an important step in reducing phosphorus losses from the farm .Growers can address these losses by matching P demand in plants with fertility management, keeping P concentrations in soils at agronomically responsive levels , and managing irrigation to minimize or eliminate runoff. It is important to note that many growers on the Central Coast and throughout the state have already initiated practices to reduce the loss of phosphorus from their farms. The University of California has several research projects in progress to document the impacts of changes in farm management, and a number of government agencies and NGOs are working with growers to improve water quality .Nitrate contamination of freshwater resources from agricultural regions is an environmental and human health concern worldwide . In agriculturally intensive regions, it is imperative to understand how management practices can enhance or mitigate the effect of nitrogen loading to freshwater systems. In California, managed aquifer recharge on agricultural lands is a proposed management strategy to counterbalance unsustainable groundwater pumping practices. Agricultural managed aquifer recharge is an approach in which legally and hydrologically available surface water flows are captured and used to intentionally flood croplands with the purpose of recharging underlying aquifers . AgMAR represents a shift away from the normal hydrologic regime wherein high efficiency irrigation application occurs mainly during the growing season. In contrast, AgMAR involves applying large amounts of water over a short period during the winter months. This change in winter application rates has the potential to affect the redox status of the unsaturated zone of agricultural regions with implications for nitrogen fate and transport to freshwater resources. Most modeling studies targeting agricultural N contamination of groundwater are limited to the root zone; these studies assume that once NO3 – has leached below the root zone, it behaves as a conservative tracer until it reaches the underlying groundwater or, these studies employ first order decay coefficients to simplify N cycling reactions . However, recent laboratory and field-based investigations in agricultural systems with deep unsaturated zones have shown the potential for N cycling, in particular denitrification, well below the root zone . For example, Haijing et al. found denitrifying enzyme activity as deep as 12 metersin an agriculturally intensive region in China. Lind and Eiland reported N2O production in sediments taken from 20 meter deep cores. Other studies have reported the capability of deep vadose zone sediments to denitrify in anerobic incubations with or without the addition of organic carbon substrates . Moreover, in agricultural settings, especially in alluvial basins such as in California with a history of agriculture, large amounts of legacy NO3 – has built up over years from fertilizer use inefficiencies and exists within the deep subsurface . It is not yet clear how this legacy nitrogen may respond to changing hydrologic regimes and variations in AgMAR practices, and more importantly, if flooding agricultural sites is enhancing nitrate transport to the groundwater or attenuating it by supporting in situ denitrification. Denitrification rates in the subsurface have been reported to vary as a function of carbon and oxygen concentrations, as well as other environmental factors . While total soil organic carbon typically declines with depth , dissolved organic carbon can be readily transported by water lost from the root zone to deeper layers and can therefore be available to act as an electron donor for denitrification . Oxygen concentration in the vadose zone is maintained by advective and diffusive transport, while oxygen consumption occurs via microbial metabolic demand and/or abiotic chemical reactions . The effects of drying and wetting cycles on oxygen concentrations in the deep subsurface are not well documented. However, in 1 meter column experiments, there is some evidence that O2 consumption proceeds rapidly as saturation increases and rebounds quickly during dry periods . These variations in oxygen concentration can influence N cycling and thus, transport to groundwater.