National Institute for Water Resources, Water Resources Research Institute Program
03/01/08 - 02/28/09
Tao Yan, and Clark C.K. Liu
Nutrient enrichment in rivers and lakes has been a changing phenomenon in the past three decades in the United States. Contribution from point sources, such as wastewater discharge, has been significantly controlled and reduced, whereas non-point sources, such as urban and agricultural run-offs, are increasingly blamed for water eutrophication problems across the country. Due to the complex nature of nutrient dynamics in these waterbodies, accurate and timely trophic assessment is a prerequisite for efficient and cost-effective regulatory
and remediation actions, and is a subject of intensive research.
Traditional methods used in trophic assessment can be classified as either single parameters or composite indices. The single trophic parameters, including total nitrogen (TN), total phosphorus (TP), Chlorophyll a content (Chl-a), and diurnal dissolved oxygen variation (DDO) are easy to measure, but usually do not address the multi-dimensional nature of the waterbody eutrophy. The current composite indices, such as the lake waters Carlson trophic status index (TSI) and its derivative for streams and rivers, are integrated from multiple single parameters and therefore more inclusive. These traditional methods have become indispensable tools of water resources research, in cataloging and evaluating the trend of water eutrophication.
In spite of their tremendous value in water resource management, the traditional water trophic assessment methods share a common shortcoming, in that they do not address the interactions between the abiotic factors and the biotic factors. These methods either measure the concentration of nutrients, such as TN and TP, or the overall indicator of primary production, such as Chl-a and DDO. Understanding the interaction between the abiotic and biotic factors is important, because it is essential for developing predictive capabilities of the trophic assessment methods, and how would the microbial communities respond to nutrient loading is at the core of understanding eutrophication.
Recent advancement of molecular tools, such as 16/18s RNA gene-based clone library or community fingerprinting, has enabled cultivation-independent survey of microbial species biodiversity. The molecular biodiversity, i.e. the estimated microbial species richness and evenness, is directly influenced by the interactions between biotic and abiotic factors, therefore theoretically directly relating to eutrophication. So far, a few works have been done to use molecular tools to study the microbial community in waterbodies with different tropic states, but little has been documented as to how change of trophic state affect the biodiversity, and vice versa.
The focus of this research was to investigate if microeukaryotic and prokaryotic biodiversity can be used for trophic assessment and the prediction of eutrophication in the Wahiawa Reservoir on the island of Oahu, Hawaii. The Wahiawa Reservoir is an important water source; it is the largest freshwater sport fishery in the state, and it also provides irrigation water for agricultural purposes. In the past years, the reservoir has experienced occasional eutrophication problems, such as overgrowth of Salvinia molesta weed, due to the treated effluent from the Wahiawa WTP and nutrient-rich storm runoff. How to assess the trophic state of the reservoir in a timely fashion is critical for the best management of this important water resource.
The project was an essential part of the concerted effort by the Pls in adopting modern molecular tools in water resource management. We sought to understand the causal relationships between nutrient loadings and biological responses. The traditional approaches that characterize the biological responses (i.e. Chl-a and DDO) were compared with the biodiversity-based molecular approaches. We believed that the molecular approaches, due to their capability of assessing the biological response at individual species level, would be more sensitive than the traditional overall parameters and more predictive because of the improved understanding of the microbial population dynamics.
Objective 1: Investigating the relationship between trophic status and microbial biodiversity.
The working hypothesis for this objective was that the eutrophication process, i.e. transition from oligotrophic to eutrophic and to hypertrophic, corresponds to decrease in microbial biodiversity. This was carried out in laboratory microcosms where eutrophication was artificially stimulated.
Objective 2: Field monitoring of the dynamics of trophic status and biodiversity at Wahiawa Reservoir.
The field investigation was designed to corroborate and extend Objective 1. The laboratory-stimulated trophic transition is one-dimensional, whereas under field conditions, factors from different levels play together. This field monitoring also provided important information about the population dynamics and stress responses of the microbial communities at the Wahiawa Reservoir.