Eliška Rejmánková

Linking environmental nutrient enrichment and disease emergence in humans and wildlife

Worldwide increases in human and wildlife diseases have challenged ecologists to understand how large-scale environmental changes affect host-parasite interactions. One of the most profound changes to Earths ecosystems is the alteration of global nutrient cycles, including those of phosphorus (P) and especially nitrogen (N). Along with the obvious direct benefits of nutrient application for food production, anthropogenic inputs of N and P can indirectly affect the abundance of infectious and noninfectious pathogens. The mechanisms underpinning observed correlations, however, and how such patterns vary with disease type, have long remained conjectural. Here, we highlight recent experimental advances to critically evaluate the relationship between environmental nutrient enrichment and disease. Given the interrelated nature of human and wildlife disease emergence, we include a broad range of human and wildlife examples from terrestrial, marine, and freshwater ecosystems. We examine the consequences of nutrient pollution on directly transmitted, vector-borne, complex life cycle, and noninfectious pathogens, including West Nile virus, malaria, harmful algal blooms, coral reef diseases, and amphibian malformations. Our synthetic examination suggests that the effects of environmental nutrient enrichment on disease are complex and multifaceted, varying with the type of pathogen, host species and condition, attributes of the ecosystem, and the degree of enrichment; some pathogens increase in abundance whereas others decline or disappear. Nevertheless, available evidence indicates that ecological changes associated with nutrient enrichment often exacerbate infection and disease caused by generalist parasites with direct or simple life cycles. Observed mechanisms include changes in host/vector density, host distribution, infection resistance, pathogen virulence or toxicity, and the direct supplementation of pathogens. Collectively, these pathogens may be particularly dangerous because they can continue to cause mortality even as their hosts decline, potentially leading to sustained epidemics or chronic pathology. We suggest that interactions between nutrient enrichment and disease will become increasingly important in tropical and subtropical regions, where forecasted increases in nutrient application will occur in an environment rich with infectious pathogens. We emphasize the importance of careful disease management in conjunction with continued intensification of global nutrient cycles.

Detecting seasonal flooding cycles in marshes of the Yucatan Peninsula with SIR-C polarimetric radar imagery☆

Abstract Polarimetric L- and C-band radar imagery from the shuttle imaging radar-C (SIR-C) were acquired over wetlands of the Yucatan Peninsula during the dry (April) and wet (October) seasons of 1994. Field surveys during the flights recorded biophysical data and water depth in 11 marsh sites containing communities of three principal emergent macrophytes: Cladium jamaicense, Typha domingensis, and Eleocharis cellulosa. The only major seasonal change was in flooding. Seasonal changes in polarimetric backscatter magnitude (HH, VV, and CS=(HV+VH)/2) and phase [βH-V phase differenceβ=PD) were extracted for a stable evergreen mangrove forest calibration site, which confirmed that the absolute calibration of the Yucatan imagery exceeded the SIR-C system calibration. We estimate that seasonal changes of ⩾2dB in backscatter magnitude and ⩾10° in phase (PD) are significant in our data. Seasonal changes in L- and C-band magnitude and phase were extracted from the 11 marshes, and significant changes above the calibration limit were noted. Increased flooding in the marshes was detected by: 1) an increase in backscatter magnitude in marshes with tall, dense cover; 2) a decrease in backscatter magnitude in marshes with short, sparse cover, and 3) an increase in PD in all types of marshes. Magnitude increases result from an increase in double-bounce interactions between the emergent vegetation and water surface, whereas decreases result from an increase in forward scattering off the open water. Average PD values increase owing to an absolute or relative increase in double- compared with single-bounce interaction. Changes from dry or partially flooded to completely flooded, as well as increases in water depth, could be detected by most of the polarimetric parameters, but changes from dry to partially flooded could not. C-band PD (CPD) was the radar parameter most sensitive to flooding. CPD changed significantly for all eleven marshes, followed by L-band PD (LPD) and LVV (nine marshes) and LHH, LCS, and CVV (seven marshes). CHH detected significant changes in five marshes but produced changes of ±1.8–1.9 dB (just below our estimated calibration limit) in four others. An evaluation of current spaceborne radars indicates that a combination of the European Remote Sensing Satellite (ERS-1,2) and Radarsat radars could detect seasonal flooding in a wide variety of marsh ecosystems, excluding partial flooding and flooding in small patches of short, sparse vegetation.

MONITORING PACIFIC COAST SALT MARSHES USING REMOTE SENSING

The rapid decline in the extent and health of coastal salt marshes has created a need for nondestructive methods for evaluating the condition of salt marsh ecosystems. This paper describes simultaneous uses of field sampling and remote sensing approaches to understand salt marsh ecosystem functions and species distributions and discusses the implications for salt marsh monitoring using remote sensing. Three sites along the Petaluma River near the entrance into San Pablo Bay, California, which represented a range of soil salinity, water content, and nutrients, were studied. Standing biomass was directly assessed by field sampling and indirectly estimated through canopy reflectance. The sites were dominated by almost monotypic stands of Salicornia virginica, Spartina foliosa, and Scirpus robustus. For Salicornia, we found a positive relationship between salinity and biomass up to a threshold of 42 g/kg, after which biomass declined monotonically with increasing salinity. No Scirpus or Spartina were found at soil salinities >20 g/kg. Although significantly different levels of nitrate and ammonium nitrogen were found in the interstitial water and soils at these sites, no strong relationships were found between biomass and nitrate nitrogen. Soil ammonium nitrogen, in contrast, was positively related to biomass. Soil redox and salinity increased with elevation and distance from the shoreline, while soil moisture and H2S decreased. Canopy biomass was estimable using remotely sensed spectral vegetation indices at 58–80% accuracy depending on species. Simple Vegetation Index (VI) and Atmospherically Resistant Vegetation Index (ARVI) measured by handheld field spectrometers were the best estimators of green biomass for high cover of Salicornia. Soil Adjusted Vegetation Index (SAVI) and Soil Adjusted and Atmospherically Resistant Vegetation Index (SARVI) gave the best estimates for Spartina while the Global Environment Monitoring Index (GEMI) was the best estimate for Scirpus. The relationships between vegetation indices and biomass were developed from field spectra. The VI was used to estimate spatial patterns of biomass across the salt marsh from Landsat satellite Thematic Mapper (TM) data. The TM image showed spatial patterns corresponding with species zones and biomass abundance. Narrow band reflectance features measured with a handheld spectrometer can be used to predict canopy plant water content (R2 = 63%). Interpolated estimates of water content from field-measured canopy reflectance were shown to relate to variation in salinity and soil moisture. Canopy water content was estimated from Airborne Advanced Visible Infrared Imaging Spectrometer data, which showed similar spatial patterns at the site. Results indicate that both biomass production and canopy water content can be accurately determined from remotely sensed spectral measures. Species-specific differences in these characteristics may be used for monitoring species distribution and abundance from airborne or satellite images.

A function of cyanobacterial mats in phosphorus-limited tropical wetlands

Cyanobacterial mats are important components of oligotrophic wetland ecosystems in the limestone-based regions of the Caribbean. Our goals were to: (1) Estimate the biomass and primary production of cyanobacterial mats, quantify the extent of nitrogen fixation and measure the activity of alkaline phosphatase (APA) in representative marshes of northern Belize; (2) Record changes in these variables following nutrient additions. The mat biomass ranged from 200 to 700 g m−2 AFDM, with the epipelon contributing up to 87% of the total. Tissue nitrogen was similar in all marshes (1.1–1.5%), while tissue phosphorus was extremely low (0.0055–0.0129%) and well correlated with the N:P ratio in water. Nitrogen fixation expressed as nitrogenase activity was high in some marshes (17.5 nmol C2H4 cm−2 h−1) and low (< 5 nmol C2H4 cm−2 h−1) in others depending mainly on the proportion of heterocyst-forming cyanobacteria (Nostocales, Stigonematales) in the mat. Alkaline phosphatase activity was positively correlated with the N:P ratio of the mat. Experimental addition of phosphorus resulted in significant increase in primary production and nitrogen fixation while it suppressed the APA activity. The presented data clearly showed that oligotrophic marshes of northern Belize are strongly P limited. Increased input of phosphorus would profoundly change their structure and functions.

Remote Sensing of Tropical Wetlands for Malaria Control in Chiapas, Mexico

Malaria, transmitted by anopheline mosquitoes, remains a serious health problem in the tropics. Most malaria eradication efforts focus on control of anopheline vectors. These efforts include the NASA Di-Mod project, whose current goal is to integrate remote sensing, geographic information systems (GIS), and field research to predict anopheline mosquito population dynamics in the Pacific coastal plain of Chiapas, Mexico. Field studies demonstrate that high larval production of Anopheles albimanus, the principal malaria vector in the plain, can be linked to a small number of larval habitat-types, determined by larval sampling and cluster analysis of wetlands in the coastal plain. Analysis of wet and dry season Landsat Thematic Mapper (TM) satellite imagery identified 16 land cover units within an 185-km2 study area in the coastal zone. A hierarchical approach was used to link the larval habitat-types with the larger land cover units and make predictions of potential and actual low, medium, and high anopheline production. The TM-based map and GIS techniques were then used to predict differences in anopheline production at two villages, La Victoria and Efrain Gutierrez. La Victoria was predicted to have much higher Anopheles albimanus production, based upon a 2-10 times greater extent of medium- and high-producing land cover units in its vicinity. This difference between villages was independently supported by sampling (with light traps) of adults, which were 5-10 times more abundant in La Victoria.

Effect of experimental phosphorus enrichment on oligotrophic tropical marshes in Belize, Central America.

Sedges from genus Eleocharisdominate extensive wetlands in the sugar cane growing areas of the Caribbean. Correlative data suggest that macrophytes in these wetlands are phosphorus limited. To determine effects of increased P input that can be expected, e.g. from agricultural runoff, a common sugar cane fertilizer was applied to representative plots in four marl-based and four peat-based marshes. The plots were located in the proximity of patches of Typha domingensis, which has been reported to be able to outcompete Eleocharis under nutrient rich conditions. Responses to the fertilizer treatment were documented as changes in: Plant height, density, biomass, net primary production, nutrient resorption, decomposition, plant and soil nutrient concentrations, percent cover of cyanobacterial mats, and potential colonization by Typha. Additions of phosphorus significantly increased plant density and height and, consequently, the aboveground net primary production. Phosphorus resorption efficiency following senescence was independent of fertilizer addition in Eleocharis but decreased in Typha from the fertilized plots. Phosphorus resorption proficiency was lower in fertilized plots for both Typha and Eleocharis. Decomposition of litter and cellulose assays was significantly faster in fertilized plots. No spontaneous establishment of Typha occurred in the fertilized plots, but survival of transplanted Typha was higher in fertilized plots than in controls. Increased plant density in fertilized plots led to elimination of a key component of these ecosystems, the nitrogen fixing cyanobacterial mats.

Distinguishing high and low anopheline-producing rice fields using remote sensing and GIS technologies

Abstract Worldwide, 140 million ha are devoted to rice cultivation, mostly in developing countries of the tropics and subtropics where malaria still constitutes a serious human health problem. Because rice fields are flood-irrigated on a semi-permanent basis during each growing season, they provide an ideal breeding habitat for a number of potential mosquito vectors of malaria. One of these vectors, Anopheles freeborni , is distributed throughout nearly 240 000 ha of irrigated rice in northern and central California, and may serve as a model for the study of rice field mosquito population dynamics using spectral and spatial information. Analysis of field data revealed that rice fields with rapid early season vegetation canopy development, located near livestock pastures (i.e. bloodmeal sources), had greater mosquito larval populations than fields with more slowly developing vegetation canopies located further from pastures. Remote sensing reflectance measurements of early season rice canopy development and geographic information system (GIS) measurements of distance to livestock pasture were combined to distinguish between high and low mosquito-producing rice fields. These distinctions were made with 90% accuracy nearly 2 months before anopheline larval populations peaked.

A Conceptual Framework to Develop Long-Term Ecological Research and Management Objectives in the Wider Caribbean Region

Abstract The Caribbean Sea and its watersheds show signs of environmental degradation. These fragile coastal ecosystems are susceptible to environmental impacts, in part because of their oligotrophic conditions and their critical support of economic development. Tourism is one of the major sources of income in the Caribbean, making the region one of the most ecotourism dependent in the world. Yet there are few explicit, long-term, comprehensive studies describing the structure and function of Caribbean ecosystems. We propose a conceptual framework using the environmental signature hypothesis of tropical coastal settings to develop a series of research questions for the reef–sea-grass–wetland seascape. We applied this approach across 13 sites throughout the region, including ecosystems in a variety of coastal settings with different vulnerabilities to environmental impacts. This approach follows the strategy developed by the Long Term Ecological Research program of the National Science Foundation to establish ecological research questions best studied over decades and large spatial areas.

Oxygen regime in a fishpond with duckweeds (lemnaceae) and Ceratophyllum

Abstract The depth of oxygen and the daily changes in oxygen concentration were assessed by in situ measurements in stands of (1) Lemnaceae; (2) Ceratophyllum; (3) Lemnaceae + Ceratophyllum; and (4) phytoplankton at the peak of the vegetation season. With the aim of excluding the influence of phytoplankton on the diurnal oxygen course, oxygen concentrations were measured in near-natural conditions below both the dense and the loose covers of Lemnaceae, placed in nutrient solution. The duckweeds Lemna minor L. and Spirodela polyrhiza (L.) Schleid. did not release any oxygen into the nutrient solution, despite the fact that their biomass increased by 7 and 4 gm−2 dry matter per 12 h in the loose and the dense stand, respectively. The oxygen uptake by the bottom sediments reduced the oxygen concentration by up to 2 mg1−1.

Volatile Substances from Larval Habitats Mediate Species-Specific Oviposition in Anopheles Mosquitoes

Abstract Oviposition site selection has been recognized as critical both for the survival and population dynamics of mosquitoes. Volatile substances released from larval habitats have been implicated as potential olfactory cues mediating oviposition. In our continuing studies of cues involved in oviposition site selection, we collected material from the larval habitats of Anopheles albimanus Wiedemann and Anopheles vestitipennis Dyar & Knab, i.e., cyanobacterial mats and Typha domingensis Pers. litter, respectively. The volatile compounds were extracted by freeze-drying the material and trapping the volatilized material on a −55°C titanium condenser. For oviposition trials conducted with wild-caught females, the tested volatile materials were pipetted onto filters floating on the surface of distilled water in Teflon beakers that were placed within oviposition cages. For both species, volatile materials in low concentrations increased oviposition, assessed as egg density, whereas there was a shift to reduced oviposition at higher concentrations. Volatile effect was strongly habitat/species-specific as shown by reciprocal treatment tests.