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Laguna Grande Limnology

Project Number: 2516-D2000
Project Chief: Luis R. Soler-López
Cooperators: Puerto Rico Conservation Trust
Period of Project: FY 2007-2009

Introduction

Laguna Grande view

A U S Forest Service publication highlights areas where further knowledge and research activities should be conducted to contribute to the preservation of the aquatic ecosystem (Weaver and others, 1999). Some of the research areas of interest for updated and additional information are hydrology, lagoon morphology, and updated data on lagoon water quality. Particularly important is the biological productivity of the lagoon or its ability to support the food chain. Biological productivity represents a major indicator about the health of the aquatic ecosystem. The type and degree of the biological productivity provides an indication of the predominant aquatic communities which control the first link of the lagoon’s food chain. Essentially, the hydrology, morphology, and water quality of Laguna Grande control the rate and sources of the lagoon’s productivity and aquatic ecosystem function; however, based on the literature reviewed, very little is known on these subjects, particularly with the integration of all components in a single comprehensive study.

Problem

The Laguna Grande reserve was first threatened by urban development in the mid-1960’s and, although designated a natural reserve in 1986, water quality and ecological conditions are threatened by expanding urban development within its watershed. Runoff from the Laguna Grande basin is suspected to be contributing with sewage and other contaminants from nearby sub-standard housing units without proper sewage infrastructure. During periods of high rainfall-runoff events, high nutrient loads could enter the lagoon transported by runoff which can be deleterious to the lagoon’s ecological health and impair the homeostatic capacity of the semi-enclosed coastal resource. In addition, flood tides could be contributing pollutants derived from both nearby (marinas) and distant sources (Río Fajardo) through the canal. Water quality changes could include an increase in nutrient concentrations, fecal contamination, decreased water transparency, and modification of its biotic conditions.

Objectives

The objective of this study is to obtain baseline data of the physical, chemical, biological, and bacteriological conditions of Laguna Grande to define its present characteristics for use as a baseline from which to judge future changes. The data obtained and its interpretation will provide the managers of the reserve a holistic perspective of the area’s hydrology and its relation to water-quality conditions in Laguna Grande.

Scope

The proposed study will include Laguna Grande’s main water body, the lagoon’s canal, and other adjacent areas with surface and ground water drainage to the lagoon. The lagoon and drainage area are contained within about 75 hectares. Data collection will include monthly measurements of physical, chemical, biological, and bacteriological parameters to define seasonal variability during one year. The study will also include two diel studies to determine community productivity rates, diurnal variation of selected water-quality characteristics, water interchange of the lagoon with the ocean through the outlet channel, and collection of bottom sediments for the determination of physical and chemical properties. The study is proposed to be conducted between January/February 2007 and February/March 2009, for a total of two natural years.

Strategy and Approach

The study will include the following field activities:

1. Collection of selected physical properties of water at about five stations inside the lagoon and at the canal to define monthly and seasonal variations in Laguna Grande. The parameters will include temperature, pH, dissolved oxygen, specific conductance, salinity, and Secchi disk water transparency.
2. Collect surficial water samples at four stations inside the lagoon and at the canal to define monthly and seasonal variations in fecal indicator bacteria concentrations.
3. Collect surficial water samples for the determination of total and dissolved nutrients at two stations inside the lagoon.
4. Determination of plankton productivity at two stations inside the lagoon using the light-dark bottles method.
5. Conduct diel studies twice during the year to determine the community primary productivity, the community respiration, and the community gross productivity.
6. Conduct discharge measurements at the lagoon’s canal entrance during the diel studies to quantify the inflow or outflow of water to estimate a mass balance interchange of selected constituents.
7. Collection of water samples on a monthly basis for the characterization of phytoplankton populations at two stations inside the lagoon.
8. Conduct a bathymetric survey to define the lagoon’s bottom topography referenced to mean sea level and calculate the lagoon’s water volume.
9. Collection of bottom sediments at three stations inside the lagoon to document grain size distribution, nutrient concentrations, and total organic carbon.
10. Obtain sediment core samples using standard USGS field sampling techniques (Ward and Harr, 1990) at two stations inside the lagoon to characterize sediment texture and composition and estimate sediment deposition rates by using the activity of the radioisotopes Cs-137 and Pb-210 (Webb, R.M.T., and Gómez-Gómez, Fernando, 1997; Van Metre and others, 2004).
11. Operate two tidal stations during the entire data collection period; one inside the lagoon and the other on the seaward side of the outlet channel at Las Croabas.

The proposed physical parameters, water quality, nutrients, primary productivity, phytoplankton, sediment, and tidal station locations at Laguna Grande are shown on figure 3.

Benefits

The proposed study is consistent with the mission of the USGS of providing reliable scientific information to describe and understand the Earth; manage water, biological, energy, and mineral resources; and enhance and protect our quality of life.

The proposed study is also consistent with the Puerto Rico Conservation Trust mission which is to protect and highlight the resources and natural beauties of Puerto Rico, to protect and conserve the natural resources, to develop educational programs to create conscience, and to provide resources for the enjoyment of the general public.

The proposed study is consistent with at least two of the nine priorities of the USGS Caribbean Water Science Center Science Plan:

• Ecological health of streams and coastal areas -- effects of urbanization and sub-urbanization on water resources.
• Fate and transport of contaminants -- study the effects of un-sewered households on the microbiological and chemical quality of streams and coastal resources in Puerto Rico.

This study is also consistent with one of the objectives of the USGS Strategic Science Plan (1997-2005) to maintain, provide, and improve long-term environmental and natural resources information, systematic analyses and investigations, and provide tools for decision making about natural systems.

This study will provide baseline information on the hydrology, morphology, water quality, and biological characteristics of Laguna Grande, and how these interact and affect the aquatic ecosystem. The data will provide stakeholders with a broad, rather than topic-focused overview and understanding of natural processes, which can be used to more effectively manage and monitor the Las Cabezas de San Juan Natural Reserve watershed and coastal surrounding areas to protect the reserve and promote habitat utilization.