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Scientific Investigations Report 2010–5071

Prepared in cooperation with the
Puerto Rico Environmental Quality Board for the Conservation Trust of Puerto Rico

Selected Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, Fajardo, Puerto Rico, March 2007-February 2009

By Luis R. Soler-López and Carlos R. Santos

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Abstract

Laguna Grande is a 50-hectare lagoon in the municipio of Fajardo, located in the northeasternmost part of Puerto Rico. Hydrologic, water-quality, and biological data were collected in the lagoon between March 2007 and February 2009 to establish baseline conditions and determine the health of Laguna Grande on the basis of preestablished standards. In addition, a core of bottom material was obtained at one site within the lagoon to establish sediment depositional rates.

Water-quality properties measured onsite (temperature, pH, dissolved oxygen, specific conductance, and water transparency) varied temporally rather than areally. All physical properties were in compliance with current regulatory standards established for Puerto Rico. Nutrient concentrations were very low and in compliance with current regulatory standards (less than 5.0 and 1.0 milligrams per liter for total nitrogen and total phosphorus, respectively). The average total nitrogen concentration was 0.28 milligram per liter, and the average total phosphorus concentration was 0.02 milligram per liter. Chlorophyll a was the predominant form of photosynthetic pigment in the water. The average chlorophyll-a concentration was 6.2 micrograms per liter.

Bottom sediment accumulation rates were determined in sediment cores by modeling the downcore activities of lead-210 and cesium-137. Results indicated a sediment depositional rate of about 0.44 centimeter per year. At this rate of sediment accretion, the lagoon may become a marshland in about 700 to 900 years.

About 86 percent of the community primary productivity in Laguna Grande was generated by periphyton, primarily algal mats and seagrasses, and the remaining 14 percent was generated by phytoplankton in the water column. Based on the diel studies the total average net community productivity equaled 5.7 grams of oxygen per cubic meter per day (2.1 grams of carbon per cubic meter per day). Most of this productivity was ascribed to periphyton and macrophytes, which produced 4.9 grams of oxygen per cubic meter per day (1.8 grams of carbon per cubic meter per day). Phytoplankton, the plant and algal component of plankton, produced about 0.8 gram of oxygen per cubic meter per day (0.3 gram of carbon per cubic meter per day).

The total diel community respiration rate was 23.4 grams of oxygen per cubic meter per day. The respiration rate ascribed to plankton, which consists of all free floating and swimming organisms in the water column, composed 10 percent of this rate (2.9 grams of oxygen per cubic meter per day); respiration by all other organisms composed the remaining 90 percent (20.5 grams of oxygen per cubic meter per day). Plankton gross productivity was 3.7 grams of oxygen per cubic meter per day, equivalent to about 13 percent of the average gross productivity for the entire community (29.1 grams of oxygen per cubic meter per day).

The average phytoplankton biomass values in Laguna Grande ranged from 6.0 to 13.6 milligrams per liter. During the study, Laguna Grande contained a phytoplankton standing crop of approximately 5.8 metric tons. Phytoplankton community had a turnover (renewal) rate of about 153 times per year, or roughly about once every 2.5 days.

Fecal indicator bacteria concentrations ranged from 160 to 60,000 colonies per 100 milliliters. Concentrations generally were greatest in areas near residential and commercial establishments, and frequently exceeded current regulatory standards established for Puerto Rico.

First posted May 21, 2010

For additional information contact:
Luis R. Soler-López
U.S. Geological Survey
Caribbean Water Science Center
GSA Center building 651
Federal Drive, Suite 400-15
Guaynabo, P.R. 00965

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Suggested citation:

Soler-López, L.R., and Santos, C.R, 2010, Selected Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, Fajardo, Puerto Rico, March 2007-February 2009: U.S. Geological Survey Scientific Investigations Report 2010–5071, 51 p., available at http://pubs.usgs.gov/sir/2010/5071/.


Contents

Abstract

Introduction

Purpose and Scope

Description of Study Area

Methods of Investigation

Bathymetric Survey

Surface-Water and Rainfall Data Collection

Water-Quality Sampling and Analysis

Determination of Biological Characteristics

Sediment Core Collection

Bathymetry

Selected Hydrologic Characteristics

Rainfall and Runoff

Seawater Exchange

Water Quality

Field-measured properties and constituents

Temperature

Turbidity

pH

Dissolved Oxygen

Specific Conductance

Salinity

Water Transparenc

Other Chemical Constituents

Nitrogen and Phosphorus

Chlorophyll a

Biological Characteristics

Primary Productivity

Phytoplankton Net Primary Productivity, Plankton Respiration, and Plankton

Gross Productivity

Community Net Primary Productivity, Respiration, and Gross Productivity

Phytoplankton Biomass

Fecal Indicator Bacteria

Sediment Accretion

Summary and Conclusions

References



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