Where's the Grass? Disappearing Submerged Aquatic Vegetation and Declining Water Quality in Lake Mattamuskeet

Abstract

Major threats to aquatic systems such as shallow lakes can include declining water quality, the loss of macrophyte beds, and the occurrence of harmful algal blooms. Often, these changes go unnoticed until a shift from a clear, oligotrophic system dominated by macrophyte beds to a turbid, eutrophic system dominated by phytoplankton and associated harmful algal blooms has occurred. Lake Mattamuskeet, which mostly lies within the boundary of Mattamuskeet National Wildlife Refuge, North Carolina, is a shallow lake that has recently experienced a reduction in water clarity and macrophyte beds, also referred to as submerged aquatic vegetation (SAV), and an increase in nutrients, phytoplankton, harmful algal blooms, and cyanotoxin production. At Lake Mattamuskeet, SAV coverage and water clarity declined between the 1980s and 2015. During the same time, significantly increasing trends in nitrogen, phosphorus, turbidity, suspended sediments, chlorophyll a, and pH occurred. Current water-quality conditions (2012)(2013)(2014)(2015) are not conducive to SAV survival and, in some cases, do not meet North Carolina water-quality standards for the protection of aquatic life. Water clarity declines appear to predate the SAV die-offs on the east side. Moving forward, SAV will serve as a primary indicator for lake health; and lake monitoring, research, and management efforts will focus on the restoration of aquatic grasses and water quality at Lake Mattamuskeet.

A Retrospective Analysis of Agricultural Herbicides in Surface Water Reveals Risk Plausibility for Declines in Submerged Aquatic Vegetation

“…In addition to herbicide toxicity, SAV decline in the APES and elsewhere may be attributed to other factors, including decreased light penetration due to sedimentation and excess phytoplankton growth related to nutrient enrichment [ 8 , 10 , 11 ]. While herbicides in surface waters may not affect healthy plants or animals, the presence of herbicides may adversely affect sensitive species already experiencing stressful conditions such as habitat degradation or poor water quality [ 38 ].…”

“…Thus, changes in agricultural land use (i.e., specific crops grown), associated herbicide use on those crops, and measurements of herbicides in adjacent surface waters sometimes exceeding safety thresholds or guidelines are multiple lines of evidence of potential herbicide exposure and effects on SAV that were observed during this time period. In Beaufort and Hyde counties, where the percent of acres harvested for cotton rose sharply from 4 to 24% and from 6 to 23%, respectively, from 1997 to 2002 ( Figure 4 and Figure 5 ), there would have been a concomitant increase in the use cotton herbicides (and potentially growth regulators and defoliants used in cotton production), which may have adversely affected SAV in the adjacent waters and specifically in Lake Mattamuskeet where few herbicide samples have been taken and analyzed [ 11 ].…”

“…Native SAV provides critical habitat (food and cover) for many waterfowl and fish species, and its loss may be influencing their population density, diversity, and distribution, as well as overall water quality through diminished ecosystem services [ 11 , 42 ]. Many factors are known to influence the presence of SAV, and agricultural herbicide runoff and toxicity is among them.…”

“…There are many potential factors contributing to the observed declines in SAV, such as water clarity (including light penetration into the water column), water depth, sediment composition, water currents, salinity, the presence of plankton and toxic blooms, invasive species, nutrient enrichment, dissolved oxygen levels, and herbicides [ 10 , 11 ]. Among the many threats to SAV, the issues of herbicide runoff, exposure, and toxic effects are important because of the extensive agricultural production that occurs in estuarine drainage areas.…”

“…The aim of this project was to evaluate the potential role of agricultural herbicides in the decline of SAV. The specific objectives were to assess herbicide concentrations measured in surface waters of the APES collected near the time of documented SAV declines, which has been detailed as far back as 1991 [ 2 ] and as recent as the 2010s [ 8 , 10 , 11 ]; to evaluate the risk thresholds for SAV (non-target aquatic plants) with current aquatic life criteria and standards; to compare these data to current agricultural practices and pesticide loadings; and to identify potential research and management needs for conserving SAV. This evaluation illustrates the value of historic data and the perspective that they can provide in an alternative context, that is, their recently understood potential relevance to the declines of SAV that were occurring and have continued to occur in this area since the original study [ 15 ] was conducted.…”