Owen T. Lind

Sensitivity of Aquatic Ecosystems to Climatic and Anthropogenic Changes: The Basin and Range, American Southwest and México

Variability and unpredictability are characteristics of the aquatic ecosystems, hydrological patterns and climate of the largely dryland region that encompasses the Basin and Range, American Southwest and western Mexico. Neither hydrological nor climatological models for the region are sufficiently developed to describe the magnitude or direction of change in response to increased carbon dioxide; thus, an attempt to predict specific responses of aquatic ecosystems is premature. Instead, we focus on the sensitivity of rivers, streams, springs, wetlands, reservoirs, and lakes of the region to potential changes in climate, especially those inducing a change in hydrological patterns such as amount, timing and predictability of stream flow. The major sensitivities of aquatic ecosystems are their permanence and even existence in the face of potential reduced net basin supply of water, stability of geomorphological structure and riparian ecotones with alterations in disturbance regimes, and water quality changes resulting from a modified water balance. In all of these respects, aquatic ecosystems of the region are also sensitive to the extensive modifications imposed by human use of water resources, which underscores the difficulty of separating this type of anthropogenic change from climate change. We advocate a focus in future research on reconstruction and analysis of past climates and associated ecosystem characteristics, long-term studies to discriminate directional change vs. year to year variability (including evidence of aquatic ecosystem responses or sensitivity to extremes), and studies of ecosystems affected by human activity.

Clay turbidity and the relative production of bacterioplankton and phytoplankton

A comparison of phytoplankton with bacterioplanktonproduction as each ismodified by high concentrations of suspended clays ispresented. High clayturbidity caused light-limition of water columnphytoplankton production.However, the clay combined with DOC to form aggregateswhich supportedbacterioplankton production. Consequently,bacterioplankton production wasrelatively high at 42% of total small particleproduction in this lake.Bacterioplankton abundance and biomass was stronglycorrelated withphytoplankton chlorophyll a for most of the lake. Because of the association ofbacterioplankton with the clay-organic aggregates, DOCwas not a good predictorof bacterioplankton abundance or production. POC(primarily OC associatedwith clay) was correlated with bacterioplanktonabundance over most of thelake. Bacteria production was substrate limited asshown by much greaterbiomass-specific production at smaller bacteriapopulation sizes. Multipleregression analysis showed that specificbacterioplankton production wasprimarily governed by POC and secondarily by rates ofphytoplanktonproduction. Thus clay, because of light limitedphytoplankton production,negatively impacts bacterioplankton production yet atthe same time facilitatiesbacterial production by concentrating OC with theformation of the clay-organicaggregate.

Evaluation of Phytoplankton–Limiting Factors in Lake Chapala, México: Turbidity and the Spatial and Temporal Variation in Algal Assay Response

ABSTRACT Laboratory algal bioassays using both cultures of Ankistrodesmus bibraianus and natural phytoplankton, and large, in-lake, container assays with natural populations, were used to determine the factor most limiting phytoplankton production in Lake Chapala, Mexico. Both types of laboratory culture assays showed that nitrogen was the principal limiting nutrient at each station across this very large lake in all seasons. The growth response of natural phytoplankton was similar to that of A. bibraianus. However, management practices to regulate the lakes productivity based solely upon this laboratory information would be inappropriate because the natural population assays showed that the ultimate limiting factor in situ is illumination controlled by the high clay turbidity. Rarely, if ever, was phytoplankton production controlled by the laboratory-determined limiting nutrient, nitrogen, expressed in the lake. The importance of performing algal assays extensively through time and space also was demons...

Apparent bioaccumulation of cylindrospermopsin and paralytic shellfish toxins by finfish in Lake Catemaco (Veracruz, Mexico).

Compared to the well-characterized health threats associated with contamination of fish and shellfish by algal toxins in marine fisheries, the toxicological relevance of the bioaccumulation of toxins from cyanobacteria (blue-green algae), as the primary toxigenic algae in freshwater systems, remains relatively unknown. Lake Catemaco (Veracruz, Mexico) is a small, tropical lake system specifically characterized by a year-round dominance of the known toxigenic cyanobacterial genus, Cylindrospermopsis, and by low, but detectable, levels of both a cyanobacterial hepatotoxin, cylindrospermopsin (CYN), and paralytic shellfish toxins (PSTs). In the present study, we evaluated, using enzyme-linked immunoassay (ELISA), levels of both toxins in several species of finfish caught and consumed locally in the region to investigate the bioaccumulation of, and possible health threats associated with, these toxins as potential foodborne contaminants. ELISA detected levels of both CYN and PSTs in fish tissues from the lake. Levels were generally low (≤1 ng g−1 tissue); however, calculated bioaccumulation factors (BAFs) indicate that toxin levels exceed the rather low levels in the water column and, consequently, indicated bioaccumulation (BAF>1). A reasonable correlation was observed between measured bioaccumulation of CYN and PSTs, possibly indicating a mutual source of both toxins, and most likely cells of Cylindrospermopsis, the dominant cyanobacteria in the lake, and a known producer of both metabolites. The potential roles of trophic transport in the system, as well as possible implications for human health with regards to bioaccumulation, are discussed.

Interaction of water quantity with water quality: the Lake Chapala example

Water quality may be significantly determined by water quantity. Lake Chapala, Mexico is a large lake beset with numerous water quality problems. The decline in water volume over the past 20 years, a serious problem itself, is associated with causing or enhancing several problems of quality. Five such problems are explored herein. These are: extensive infestations of water hyacinth (Eichhornia crassipes), a declining native fishery, light limitation of phytoplankton production at the base of the food chain, shallow-water algal blooms resulting in water supply treatment problems, and the presence of toxic metals in the harvested and sold fishes.

The Management of Lake Chapala (México): Considerations After Significant Changes in the Water Regime

ABSTRACT Forty-five percent of the water from the river supplying Lake Chapala has been diverted in the past 10 years. The water quality consequences of this change in water quantity are numerous. In this period, the lake volume has declined by 50 percent. Because of the reduced flushing and the corresponding relative increase in outputs via evaporation more than twofold, the lakes total dissolved solids content has increased. A second consequence, and one of great importance to the ecology of the lake, is the increase in water turbidity. With a reduction in lake mean depth from 7.2 m to 4.5 m, wind-driven resuspension of the inorganic sediments has increased. This has caused light limitation of phytoplankton production to the extent that nutrient loading models to predict production of biomass fail when applied to this lake. Reduction in turbidity by increased flocculation could significantly speed up the eutrophication process, because high concentrations of nutrients are present in the system (total i...

Detecting the Increased Eutrophication Rate of Douglas Lake, Michigan: the Relative Areal Hypolimnetic Oxygen Deficit Method

ABSTRACT We use data gathered since 1971 on Douglas Lake, Michigan to illustrate the use of temperature and depth adjusted oxygen deficits to assess changes in the rate of lake eutrophication. The method of calculation with adjustments is presented. By comparing different hypolimnia in the multidepression lake, we show that localized lakeshore land use produces regional patterns of production that are detected by comparison of oxygen deficits. For some lake regions, the increase in oxygen deficit has had a linear doubling since 1971. Using historical data, this rate of change is much greater than for the previous 50 years. The oxygen deficits in the lake region farthest from lakeshore development averaged 60% of those in the region nearest lakeshore development.

Response of riverine and transition zone bacterioplankton communities to a pulsed river inflow

We investigated the impact of such a summer storm pulse on the riverine zone bacterioplankton and subsequently the transition zone bacterioplankton. Discharge varied from 0.08 m3 s−1 at the onset of the pulse to 1050 m3 s−1 at the peak. Bacterial community attributes were analyzed separately for cells free in the water and those associated with particles. Most riverine zone biomass was particle-associated during the pulse, whereas it was free-living at baseflow. Free biomass declined during the ascending arm of the hydrograph indicating washout, but 13 days later increased to an extent almost twice that present at the onset. Mean cell volumes of both free and particle-associated cells increased during the pulse with particle-associated cells larger than free-living cells. Rates of cell-specific bacterioplankton production changed in parallel with the hydrograph. In the transition zone production increased with the pulse but to a lesser extent and more slowly than in the riverine zone. Mean cell volumes of transition zone bacteria changed little with the pulse. In contrast with the riverine zone, transition zone free cells were larger than the particle-associated cells.

Trace metal concentrations in Chirostoma sp. from Lake Chapala, Mexico: Elevated concentrations of mercury and public health implications

Abstract Concentrations of six metals (Cd, Cu, Ni, Pb, Zn and Hg) were determined in Chirostoma sp. obtained from three different locations around Lake Chapala, Mexico, in July, 1996. Concentrations of all metals were below trace metal action levels, where available, with the exception of mercury. Mercury concentrations ranged from 0.217 to 8.149 μg/g dry weight, with highest concentrations in fish seined from the most turbid, eastern end of the Lake near the outflow. Samples of fried fish obtained from a fish market did not have elevated concentrations relative to other samples. High consumption of Chirostoma sp. may result in exposure to mercury that exceeds US and International guidelines. The results of this study suggest the need to evaluate mercury concentrations in all edible fish species from Lake Chapala in order to accurately assess health risks to consumers.

Trophic interactions among sympatric zooplanktivorous fish species in volume change conditions in a large, shallow, tropical lake

Significant reductions in the water volume of shallow lakes impose a restriction on species segregation promoting more interactions in the trophic relationships. The diets of three closely related zooplanktivorous silversides belonging to the Atherinopsidae species flock of lake Chapala , Mexico, were analyzed at two sites (Chirostoma jordani, C. labarcae, and C. consocium). Diets were described in critical shallow (August 2000) and volume recovery conditions (August 2005). Diets included mainly cladocerans (Bosmina, Ceriodaphnia, and Daphnia) and copepods (Cyclops). A significant difference in diets was detected when comparing years (MRPP analysis, A = 0.22, p < 0.0001) and sites at different years (MRPP analysis, A = 0.17, p = 0.004). According to niche breadth mean values, species were classified as specialized and intermediate feeders. In shallow conditions, the small range of niche breadth (1.72 to 3.64) and high diet overlap values (D = 0.64, L = 8.62) indicated a high potential for interspecific exploitative interaction. When the lake volume recovered, an increase in the niche breadth range (1.04 to 4.96) and low niche overlap values (D = 0.53, L = 2.32) indicated a reduction of the species interaction. The Mann-Whitney U-test supported this pattern by showing a significant difference between years for niche overlap (p = 0.006). The increased interaction during the low volume suggests alternative segregation in life-history variations and other niche dimensions such as spatial or temporal distribution.