USGS

Water Resources of Colorado

Limnology of Blue Mesa, Morrow Point, and Crystal Reservoirs, Curecanti National Recreation Area, during 1999, and a 25-Year Retrospective of Nutrient Conditions in Blue Mesa Reservoir, Colorado

by Nancy J. Bauch, U.S. Geological Survey, and Matt Malick, National Park Service

Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Water-Resources Investigations Report 02-4199, 101 p., 32 figs.

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Abstract

The U.S. Geological Survey and the National Park Service conducted a water-quality investigation in Curecanti National Recreation Area in Colorado from April through December 1999. Current (as of 1999) limnological characteristics, including nutrients, phytoplankton, chlorophyll-a, trophic status, and the water quality of stream inflows and reservoir outflows, of Blue Mesa, Morrow Point, and Crystal Reservoirs were assessed, and a 25-year retrospective of nutrient conditions in Blue Mesa Reservoir was conducted. The three reservoirs are in a series on the Gunnison River, with an upstream to downstream order of Blue Mesa, Morrow Point, and Crystal Reservoirs. Physical properties and water-quality samples were collected four times during 1999 from reservoir, inflow, and outflow sites in and around the recreation area. Samples were analyzed for nutrients, phytoplankton and chlorophyll-a (reservoir sites only), and suspended sediment (stream inflows only). Nutrient concentrations in the reservoirs were low; median total nitrogen and phosphorus concentrations were less than 0.4 and 0.06 milligram per liter, respectively. During water-column stratification, samples collected at depth had higher nutrient concentrations than photic-zone samples. Phytoplankton community and density were affected by water temperature, nutrients, and water residence time. Diatoms were the dominant phytoplankton throughout the year in Morrow Point and Crystal Reservoirs and during spring and early winter in Blue Mesa Reservoir. Blue-green algae were dominant in Blue Mesa Reservoir during summer and fall. Phytoplankton density was highest in Blue Mesa Reservoir and lowest in Crystal Reservoir. Longer residence times and warmer temperatures in Blue Mesa Reservoir were favorable for phytoplankton growth and development. Shorter residence times and cooler temperatures in the downstream reservoirs probably limited phytoplankton growth and development. Median chlorophyll-a concentrations were higher in Blue Mesa Reservoir than Morrow Point or Crystal Reservoirs. Blue Mesa Reservoir was mesotrophic in upstream areas and oligotrophic downstream. Both Morrow Point and Crystal Reservoirs were oligotrophic. Trophic-state index values were determined for total phosphorus, chlorophyll-a, and Secchi depth for each reservoir by the Carlson method; all values ranged between 29 and 55. Only the upstream areas in Blue Mesa Reservoir had total phosphorus and chlorophyll-a indices above 50, reflecting mesotrophic conditions. Nutrient inflows to Blue Mesa Reservoir, which were derived primarily from the Gunnison River, varied on a seasonal basis, whereas nutrient inflows to Morrow Point and Crystal Reservoirs, which were derived primarily from deep water releases from the respective upstream reservoir, were steady throughout the sampling period. Total phosphorus concentrations were elevated in many stream inflows. A comparison of current (as of 1999) and historical nutrient, chlorophyll-a, and trophic conditions in Blue Mesa Reservoir and its tributaries indicated that the trophic status in Blue Mesa Reservoir has not changed over the last 25 years, and more recent nutrient enrichment has not occurred.


Table of Contents

Foreword

Abstract

Introduction

Purpose and Scope

Acknowledgments

Description of Study Area

Precipitation and Reservoir Operations

Previous Investigations

Data Collection and Analysis Procedures

Sample Collection

Analysis Methods

Laboratory Analysis for Nutrients and Suspended Sediment

Nutrient Ratios and Loads

Water-Quality Standards

Phytoplankton and Chlorophyll-a Analysis

Reservoir Classification Schemes

Trend and Correlation Analysis

Quality-Control Methods and Analysis

Limnology of Blue Mesa Reservoir

Physical Properties

Nutrients

Phytoplankton

Chlorophyll-a

Characteristics of Reservoir Inflows

Physical Properties and Suspended Sediment

Nutrients

Characteristics of Reservoir Outflow

Limnology of Crystal Reservoir

Physical Properties

Nutrients

Phytoplankton

Chlorophyll-a

Characteristics of Reservoir Inflows

Physical Properties and Suspended Sediment

Nutrients

Characteristics of Reservoir Outflow

Comparison of Reservoirs, Inflows, and Outflows

Physical Properties in Reservoirs

Nutrients in Reservoirs

Phytoplankton in Reservoirs

Chlorophyll-a in Reservoirs

Characteristics of Reservoir Inflows and Outflows

Implications for Management

Retrospective of Nutrient Conditions in Blue Mesa Reservoir

Physical Properties

Nutrients

Phytoplankton

Chlorophyll-a

Nutrient Enrichment

Summary

References

Supplemental Data

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