Steven Arthur Loiselle

Functioning and dynamics of wetland vegetation of Lake Victoria: an overview

The aquatic macrophytic vegetation constituting the wetlands situated along the coast of Lake Victoria provides valuable services to both local and regional communities as well as an important ecological function through the transition between terrestrial and aquatic ecosystems. The wetland vegetation is typically rooted in the substrate on the landward side of the lake, but forms a floating mat towards the middle of the wetland and at the wetland/lake interface. Cyperus papyrus and Miscanthidium violaceum vegetation typically dominate the permanently inundated wetland areas along most of the shores of Lake Victoria. Due to the prevailing climatic and hydrological catchment conditions, these macrophytic plants (papyrus in particular) tend to exhibit high net productivity and nutrient uptake which strongly influences both wetland status and lake water quality. In addition, these wetlands provide important economic livelihoods for the local populations. The integrity and physical structure of these wetlands strongly influences their associated mass transport mechanisms (water, nutrients and carbon) and ecosystem processes. Wetland degradation in Africa is an increasing problem, as these ecosystems are relied upon to attenuate industrial, urban and agricultural pollution and supply numerous services and resources. In an integrated project focused on the wetlands of Lake Victoria, the ecological and economic aspects of littoral wetlands were examined and new instruments developed for their sustainable management.

The Spatial Distribution of Optical Properties in the Ultraviolet and Visible in an Aquatic Ecosystem

Abstract In aquatic ecosystems, the UV and visible radiation environment is strongly influenced by variation in the chemical and physical parameters of the ecosystems. In shallow lakes, highly heterogeneous water characteristics produce a wide variety of optical environments. Such ecosystems require analysis approaches that consider a potential variability. In this study, 77 stations were used to characterize the optical properties of a shallow lake (open water surface 54 km2). The vertical attenuation of solar radiation at 305, 313, 320 and 340 nm and at photosynthetically active radiation was measured during the seasonal cycle. Dissolved organic matter (DOM), turbidity, fluorescence, pH, temperature, conductance and dissolved oxygen were simultaneously measured. The spatial variation of the extinction spectra of the dissolved fraction at each sampling station was also measured and analyzed between 270 and 400 nm. The spatial heterogeneity of the lake was examined by determining the distributions of the attenuation coefficients and biooptical parameters at high spatial resolution and describing the distributions in a series of maps. The methodology permitted a quantitative description of the interaction between solar radiation and aquatic ecosystems as well as a spatial classification of the dominating processes within the lake. This included the determination of the role played by DOM loading and changing chemical properties within the lake optical environment.

A new three-band algorithm for estimating chlorophyll concentrations in turbid inland lakes

A new three-band model was developed to estimate chlorophyll-a concentrations in turbid inland waters. This model makes a number of important improvements with respect to the three-band model commonly used, including lower restrictions on wavelength optimization and the use of coefficients which represent specific inherent optical properties. Results showed that the new model provides a significantly higher determination coefficient and lower root mean squared error (RMSE) with respect to the original model for upwelling data from Taihu Lake, China. The new model was tested using simulated data for the MERIS and GOCI satellite systems, showing high correlations with the former and poorer correlations with the latter, principally due to the lack of a 709 nm centered waveband. The new model provides numerous advantages, making it a suitable alternative for chlorophyll-a estimations in turbid and eutrophic waters.

Chemical and optical phototransformation of dissolved organic matter

Dissolved organic matter represents the main reservoir of organic carbon in most aquatic ecosystems. In the present study, we determined the optical changes and the quantum yields of transient species formation for chromophoric dissolved organic matter (CDOM) samples undergoing photodegradation. The results show that the triplet states (3)CDOM* are potentially key players in CDOM photodegradation and that such transformations are strongly influenced by small differences in CDOM sources and sinks. In contrast, ·OH radicals are very unlikely to play a key role in phototransformation. These results represent an important first step in combining optical and transient species analyses to understand photodegradation processes of dissolved organic matter.

Assessing the optical changes in dissolved organic matter in humic lakes by spectral slope distributions

The impact of photodegradation and mixing processes on the optical properties of dissolved organic matter (DOM) was examined using a distribution of absorption spectral slopes and fluorescence measurements in two Argentine lakes. By examining the variability of the absorption spectral slopes throughout the ultraviolet and visible wavelengths, it was possible to determine which wavelength intervals were most sensitive to dominant loss processes. For DOM photodegradation, results show that increases in the absorption spectral slope between 265 and 305 nm were highly sensitive to increased exposure to solar ultraviolet radiation. A slightly larger wavelength range (265-340 nm) was found to be influenced when both mixing and photodegradation processes were considered, in terms DOM residence time, DOM absorption and UV diffuse attenuation coefficients. This same interval of spectral slopes (265-340 nm) was found to highly correlate with changes in fluorescence emission/excitation in wavelengths that are typically associated with terrestrial humic-like DOM. The identification of specific wavelength intervals, rather than the use of standard wavelength intervals or ratios, improved our ability to identify the dominant dissolved organic matter (humic-like) and major loss mechanisms (photodegradation) in these lakes.

Fourteen-Year Record (2000–2013) of the Spatial and Temporal Dynamics of Floating Algae Blooms in Lake Chaohu, Observed from Time Series of MODIS Images

As the fifth largest freshwater lake in China, Lake Chaohu has drawn increasing attention due to the decline in water quality and the occurrence of massive algal blooms. We applied an algae pixel-growing algorithm to MODIS Terra or Aqua data (2100 images) to characterize surface floating algae bloom dynamics from 2000 to 2013 with respect to meteorological and lake nutrient conditions. The results show an increase in surface algal bloom coverage, frequency, and duration with a trend toward earlier bloom formation. Importantly, spatial and temporal patterns in the historically less compromised eastern and middle lake areas show that water quality conditions are deteriorating. This has occurred at the same time as lake management has made a catchment scale effort to reduce impact. Our results show that nutrient concentrations were not the main driver of inter-annual bloom variations. Local meteorological conditions, in particular wind speed and temperature, played an important role in the dynamics of floating algal bloom. This highlights the important challenges for lake management.

Modelling energy fluxes in remote wetland ecosystems with the help of remote sensing

The study of the material and energy fluxes that characterise an ecosystem is fundamental to understanding the behaviour of that system and its resiliency in the face of changing local and global conditions. In the present research, an extended energy balance model was used to examine the fluxes that characterise the energy exchange between a large wetland area and its environment. For each flux term, a number of modelling approaches was compared. Modelling was complemented through the use of irradiance measurements from satellite data. The model output shows the daily and season variations in each energy term, as well as a comparison of the modelling approaches. The final output of the model is validated with the measured hourly variation in the enthalpy (water temperature) of the lagoon. A comparison is made of the relative importance of each energy term in the overall balance of the system. This physical based model is being used to examine possible modifications of local and global environments to the wetland energy balance. The model is also being combined with fauna models to examine the relation between climate and population characteristics (habitat). The present work is directed towards the study of a subtropical lagoon in the internationally important wetland, the Esteros del Ibera in Argentina.

Basin-scale control on the phytoplankton biomass in Lake Victoria, Africa.

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria.

Variability of particulate organic carbon in inland waters observed from MODIS Aqua imagery

Surface concentrations of particulate organic carbon (POC) in shallow inland lakes were estimated using MODIS Aqua data. A power regression model of the direct empirical relationship between POC and the atmospherically Rayleigh-corrected MODIS product (Rrc,645-Rrc,1240)/(Rrc,859-Rrc,1240) was developed (R2?=?0.72, RMSE?=?35.86 ?gL?1, p?<?0.0001, N?=?47) and validated (RMSE?=?44.46 ?gL?1, N?=?16) with field data from 56 lakes in the Middle and Lower reaches of the Yangtze River, China. This algorithm was applied to an 11 year series of MODIS data to determine the spatial and temporal distribution of POC in a wide range of lakes with different trophic and optical properties. The results indicate that there is a general increase in minimum POC concentrations in lakes from middle to lower reaches of the Yangtze River. The temporal dynamics of springtime POC in smaller lakes were found to be influenced by local meteorological conditions, in particular precipitation and wind speed, while larger lakes were found to be more sensitive to air temperature.

Feedback analysis in reserve management: studying local myths using qualitative models.

A qualitative modelling approach is used to highlight potential feedback mechanisms and secondary effects of management actions on a number of key species in a large Argentine wetland. The development of this qualitative model, in this case a loop model, was a first step in the creation of an overall management and monitoring programme for this important wetland. The use of loop models is a possible strategy in environmental management and monitoring programmes of large complex natural reserves, in which little quantitative data is available. Based upon observed ecological information, several loop trophic models were constructed and appropriate links made to reflect actual trophic relationships between trophic variables. The potential secondary effects of a hunting ban on caiman in a large shallow lagoon were examined. Indirect secondary effects were found to produce unforeseen negative impacts on other predator species (piscivorous birds, piranha) and species important to the abiotic environment (large bottom dwelling fish). As there are limited quantitative data, field investigations and the development of other types of models are necessary to examine in detail the quantitative variations on the populations in question. The results of this model were found useful to indicate where further study and quantitative data gathering might be best focused.