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

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

4noThe 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 ArgentinareservedmixedLoiselle, S.A.; Bracchini, L.; Bonechi, Claudia; Rossi, ClaudioLoiselle, S. A.; Bracchini, L.; Bonechi, Claudia; Rossi, Claudi

Neotropical wetlands: new instruments in ecosystem management

The management of wetland ecosystems in the neotropics is a challenging task due to the ecological, geographical and socio-political complexity of these areas. These difficulties are compounded by an incomplete knowledge of system dynamics and the potential impacts that economic activities may have on wetland ecosystem functioning. There is a growing body of potential tools that can be utilized to study and monitor these large complex ecosystems. These include the use of remote sensing information, in situ hydrological and microclimate sensors with long-term dataloggers, hydrological and ecological modelling approaches based on limited historical data, energy-based models, ecological-economic indicators of wetland resource use and others. We tested some of these approaches in an international research project dedicated to the study of a large Argentine wetland, the "Esteros del Ibera" (www.unisi.it/ wetland). The study focused on examining and developing instruments for the management of Latin American wetlands. Local and regional decision makers are now using project-generated instruments to make decisions for wetland resource management. The creation of innovative decision-making tools for sustainable wetland-resource utilization is an important challenge for the future

Sensitivity analysis of CDOM spectral slope in artificialand natural samples: an application in the central easternMediterranean Basin

In the past two decades, optical properties of chromophoric dissolved organic matter (CDOM) in marine environments have been extensively studied. Many of these studies report CDOM properties for the offshore environment where this complex mixture of optically active compounds is strongly diluted. Nevertheless, autochthonous and allochthonous sources have been identified and sinks related to photodegradation and bacterial activity have been demonstrated. The calculation of the spectral slope of the CDOM absorption curve has been proven to be useful and is often reported. However, a rigorous uncertainty analysis of the slope calculation is rarely reported. In this paper, we propose a method to evaluate the uncertainty of CDOM spectral slope calculated between 270 and 400 nm, using both naturally sampled and artificial solutions. We use these results to study the ultra-oligotrophic waters of the Mediterranean Sea (central eastern basin), where little is known about CDOM spatial distribution. We show that dilutions of both artificial and natural samples produce a Gaussian distribution of spectral slopes, indicating that consistent values may be determined, with a typical uncertainty of ±0.0004 nm-1 when absorption at 300 nm was greater then 0.1 m-1 (0.1 m pathlength). Comparing the distribution of spectral slopes from central eastern basin samples to a Gaussian distribution, we show differences between measurements that were significantly different. These values allow us to distinguish possible sources (algal derived CDOM), sinks (e.g. photo-bleaching) at different depths. We propose a subdivision of CDOM compounds into refractory and semilabile/refractory pools and evaluate the CDOM spectral slope of algal derived CDOM released at or near deep chlorophyll maximum

Modelling Upwelling Irradiance using Secchi disk depth in lake ecosystems

A simple model for upwelling irradiance has been developed. The model represents the relationship between Photosynthetically Active Radiation diffuse attenuation coefficients and Secchi disk depth described with a physical-mathematical expression. This physical mathematical expression allows the evaluation of the sub surface upwelling irradiance that was generated by the interaction between downwelling irradiance and the water column. The validation of the relation was performed using experimental data collected from five different aquatic ecosystems at different latitudes, solar elevations and irradiance levels. We found a good linear, positive correlation between the theoretical and measured upwelling irradiance (R2 = 0.96). The residues were well distributed, around the null value, according a Gaussian curve (R2 = 0.92). The results confirm the importance and the versatility of the Secchi disk measurements for aquatic optics

Factors influencing adoption of crop and forage related and animal husbandry technologies by small-scale dairy farmers in the highlands of central Mexico

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Competition for spectral irradiance between epilimnetic optically active dissolved and suspended matter and phytoplankton in the metalimnion. Consequences for limnology and chemistry,

In deep lakes, water column stratification isolates the surface water from the deeper bottom layers, creating a three dimensional differentiation of the chemical, physical, biological and optical characteristics of the waters. Chromophoric dissolved organic matter (CDOM) and total suspended solids (TSS) play an important role in the attenuation of ultraviolet and photosynthetically active radiation. In the present analysis of spectral irradiance, we show that the wavelength composition of the metalimnetic visible irradiance was influenced by epilimnetic spatial distribution of CDOM. We found a low occurrence of blue-green photons in the metalimnion where epilimnetic concentrations of CDOM are high. In this field study, the spatial variation of the spectral irradiance in the metalimnion correlates with the observed metalimnetic concentrations of chlorophyll a as well as chlorophyll a: chlorophyll b/c ratios. Dissolved oxygen, pH, and nutrients trends suggest that chlorophyll a concentrations were representative of the phytoplankton biomass and primary production. Thus, metalimnetic changes of spectral irradiance may have a direct impact on primary production and an indirect effect on the spatial trends of pH, dissolved oxygen, and inorganic nutrients in the metalimnion

Spatial dynamics of chromophoric dissolved organic matter in nearshore waters of Lake Victoria

The underwater light conditions in the African Great Lakes depend on the complex dynamics of ecological and hydrological forces, and are strongly influenced by local environmental conditions and global biogeochemical cycles. Changes in the optical conditions in these lakes have direct impacts on ecosystem productivity, carbon dynamics and nutrient availability. A central role in the underwater light climate is played by dissolved organic matter which is present in all aquatic ecosystems. The chromophoric fraction of these compounds can mediate ecosystem change through its influence on the attenuation of ultraviolet and PAR radiation, microbial carbon cycling and radiative transfer. In the African Great Lakes, little information is available regarding the dynamics of dissolved organic matter and those sources and sinks which control its presence in the water column. We present an extensive spatial analysis of three major bays on the Kenyan and Ugandan shores of Lake Victoria. We use these data to examine the dynamics of chromophoric dissolved organic matter in different bays and we develop a model to estimate its flow from these bays to the Lake, considering both conservative mixing and photodegradation processes. While some bays release chromophoric dissolved organic matter practically unmodified into the Lake, increased residence time and exposure to solar ultraviolet radiation create conditions where chromophores are lost before entering the open lake

Temporal variability of water temperature (<i>LST</i>) and net heat flux (<i>NHF</i>) in the surface layer of Lake Victoria.

<p><i>LST</i> time series for North Region (A) and South Region (B) indicated by a solid line. The grey line represents the corresponding monthly variability of OC4v4-<i>chl</i> for each Region. Scaling is the same for both graphs (0–44 mg m⁻³). (C) <i>NHF</i> into the surface layer in the North (solid line) and South (dashed line) Regions. NHF was significantly related with the monthly <i>LST</i> variation (<i>LST_t – LST_t−1</i>) (R = 0.651, p<0.001, n = 158). Black bars in the lower axes of figures A and B indicate periods of bloom collapse during the seasonal warm phase.</p