Abstract
Point and nonpoint sources of phosphorus (P) and nitrogen (N) can cause reductions in water quality, including eutrophication. Nonpoint pollution represents a special challenge because of dispersed not easily identifiable sources such as the runoff from soil, nutrients, and other chemicals from agricultural fields and residential areas. Laguna Cartagena is a tropical freshwater wetland, situated in southwestern Puerto Rico. It is a eutrophic ecosystem, and its eutrophication is caused by both external nutrient loading and internal, mainly by phosphorus. This wetland has been affected by phosphorus loading from inorganic agricultural fertilizer in this historically oligotrophic wetland system until the end of subsidized fertilizer use and sugar cane cultivation in the late 1990s. This study identifies: 1) nonpoint sources of phosphorus (SRP, Soluble Reactive Phosphorus and TP, Total Phosphorus) and nitrogen (nitrate, nitrite, and ammonia) that enter Laguna Cartagena; and 2) the role of precipitation events on the contributions of phosphorus and nitrogen loading to ecosystems. Herein we assess water samples from five channelized external sources of P and N that enter Laguna Cartagena at two-week intervals from October 2013 through November 2014. Rainfall data were obtained weekly from a rain gauge. Standard methods were used for all chemical analyses. Results showed that the channelized waterways that carry water to the lagoon can be classified as hypereutrophic (>100 μg/L) for TP concentrations and oligotrophic (<200 μg/L) for nitrogen concentrations. Currently agriculture (rice and cattle) is the predominant land use at the nearby University of Puerto Rico (UPR) Lajas Agricultural Experiment Substation, the predominant nonpoint source of nutrient pollution (SRP, TP and ammonia) in the principal channelized water sources to the lagoon. Current nutrient loads are likely derived from fertilizers applied to the Substation’s rice fields, and a high density livestock. The second important cause of external surface water degradation (SRP, TP and ammonia) is the discharge from rural households in the drainage basin that discharge greywater directly to the environment, as indicated by the results from Cerro Alto hills immediately to the north of the lagoon. Precipitation also was associated with SRP, TP and ammonia loads.
Highlights
Wetlands are often considered “the kidneys of the watershed” because they provide many services to society, such as water quality improvement, recharging of water supplies including groundwater reservoirs, the buffering of river and stream discharge, and microclimate regulation [1] [2]
The University of Puerto Rico (UPR) Agricultural Experimental Substation Drainage Canal (SRP mean 443.5 μg/L and total phosphorus (TP) mean 2469.3 μg/L) and Cerro Alto northern hill sides (SRP mean 310.2 μg/L and TP mean 1626.2 μg/L) provided the highest concentrations of SRP and TP (Figure 3) with Cerro Alto exhibiting the highest concentrations of phosphorus (P) in periods of high rainfall
The water quality results in this study showed that dissolved oxygen (DO) levels measured in all canals were more than 2 mg/L, (13 of 17 occasions)
Summary
Wetlands are often considered “the kidneys of the watershed” because they provide many services to society, such as water quality improvement, recharging of water supplies including groundwater reservoirs, the buffering of river and stream discharge, and microclimate regulation [1] [2]. Wetlands are characterized by their sediment retention, flood prevention, organic carbon and nutrient storage, timber production, the provision of non-timber products, medicinal plants, biodiversity support, providing drinking water for humans and livestock and offering opportunities for transport, recreation, ecotourism, and research [1]. Wetlands have been drastically altered by human disturbances, causing reductions in water quality including severe cultural eutrophication. Anthropogenic land use, due to economic development and population growth, influence phosphorus (P) and nitrogen (N) loads to waterbodies through both point and nonpoint sources [3], which can lead to eutrophication of surface water. Point sources of pollution are those which have a direct identifiable source, while nonpoint sources of pollution are those which arrive from various dispersed sources, such as the runoff of soil and sediments, nutrients, and other chemicals from agricultural fields, residential areas, and other lands [4] [5]. Nonpoint sources are difficult to control due to their diffuse nature that can be influenced by weather conditions [4]
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