On the presence of Pseudo-nitzschia calliantha Lundholm, Moestrup et Hasle and Pseudo-nitzschia delicatissima (Cleve) Heiden in the Southern Adriatic Sea (Mediterranean Sea, Italy)

This investigation included a 2-year monitoring program aimed at assessing the abundance and distribution of harmful marine phytoplankton along the Southern Adriatic coast of Italy. Monthly sampling was conducted from April 1995 to March 1997 at four sampling stations along four transects, to determine the temporal and spatial presence/abundance of the potentially toxic Pseudo-nitzschia species. The study focussed on the most abundant taxa Pseudo-nitzschia calliantha and Pseudo-nitzschia delicatissima, the identities of which were confirmed by TEM on cleaned net material. The distribution patterns of these potential Amnesic Shellfish Poisoning (ASP) toxin producers were statistically analysed by Generalised Linear Model ANalysis Of VAriance, Principal Component Analysis (GLM ANOVA, PCA) and Spearman's correlation analysis in order to address relationships between environmental variable and population dynamics. Abundances displayed horizontal and vertical structure in the study area. Inter-annual variability was also observed for both species that appeared to respond differently to the environmental factors investigated. Distributions of P. calliantha showed a stronger seasonality and was more correlated with winter water conditions than P. delicatissima, which in turn exhibited a broader temporal distribution and appeared independent from major environmental constraints. This is the first report of the occurrence and dynamics of P. calliantha and P. delicatissima populations in Southern Adriatic coastal waters

Light limitations to algal growth in tropical ecosystems

1. Spatial and temporal variations in algal concentrations are controlled in many aquatic ecosystems by the availability of solar irradiance, rather that nutrients or grazing. In such light limiting conditions, changes in the optical or hydrological characteristics of the water column will directly impact biomass concentrations. Here we develop and test an approach based on the relationship between available solar irradiance within the mixed layer and algal biomass concentrations. 2. As with most nutrient/biomass relationships, an increase in available solar energy favours an increase in biomass when light limitation prevails. The ratio between light/biomass is then used to determine a critical light requirement that can be used to estimate critical depth and compensation irradiance and open the way to exploring how changes in mixing depth and vertical attenuation may influence algal biomass concentrations. 3. This approach is used to describe real conditions in two disparate algal communities; the phytoplankton community in Lake Victoria, East Africa and the microphytobenthos community in the lacustrine system of Esteros del Iberá (South America). 4. Differences in the critical light requirement were used to examine the relative efficiency of the algal communities in their use of available solar energy. The tropical phytoplankton community showed similar energetic requirements to theoretical estimates and were found to be less efficient when compared with the phytobenthos community

Characterization of the Ugandan inshore waters of Lake Victoria based on temperature-conductivity diagrams

Temperature-conductivity diagrams are shown as a valid instrument to analyze the hydrographic structure of freshwater ecosystems, even along the surface waters. We put this method in practice in the Ugandan inshore waters of Lake Victoria. A complementary parameter (T-C anomaly) was used to differentiate between upland water intrusions. The relative value of the T-C anomaly provided information about the nature of the water intrusions and showed a considerable correlation with the biological characteristics of the water masses. The results indicated that the connections between catchment attributes, water characteristics, and biological community are quite direct in the inshore waters of Lake Victori

Effective upwelling irradiance depths in turbid waters: a spectral analysis of origins and fate

The spectral distribution of upwelling and downwelling irradiance were used to estimate the effective upwelling irradiance depth as well as examine the angular distribution of the downwelling radiance. The effective upwelling depth was seen to undergo spectral “shifts” in wavelength maxima in relation to elevated particulate concentrations. Wavelengths of the UVA minimum and mid visible maximum depths were found to be shifted to higher wavelengths (red shifted) at high particulate concentrations, while expected minimums at chlorophyll and phycocyanin absorption peaks and in the NIR were shifted to lower wavelengths (blue shifted). By comparing upwelling and downwelling irradiance profiles, the wavelength limits of the asymptotic angular radiance distribution were found to correspond to the visible spectral domain (390 – 740 nm)

The analysis of the seasonal, spatial and compositional distribution of humic substances in a subtropical shallow lake

The spatial and temporal distribution of humic substances in aquatic ecosystems can have important effects on ecosystem productivity, negatively impacting primary productivity while positively impacting secondary productivity. In the present investigation, a large shallow lake ecosystem was studied to determine the spatial and seasonal variation of the Hydrobiology, composition and concentration of humic substances. Concentrations of total dissolved orMinister of Agriculture, ganic matter, humic acid, and fulvic acid were found to display significant spatial distributions (1.3...13.5 mg/L, DOM; 0.1...5.4 mg/L, HA). The distribution is described by using mapping techniques and the analysis of the spatial distribution of the lake. An analysis of the seasonal variations also indicated the dependence of the occurrence of these compounds on meteorological and hydrological conditions. To identify the potential sources of these organic materials, an analysis was made of the ratio of humic and fulvic acid fractions and total DOM. It was found that areas of high DOM concentration coincided with the areas of highest HA percentage of total DOM. Furthermore using the ratio of the normalised concentrations of HA, FA, and residual DOM (< 5000 g/mol) it was found that areas dominated by each are spatially distinct. This confirms the hypothesis that in these shallow lakes, photodegradation and bacterioplankton activity will create a residence time dependent zonation of each component of the total DOM

The spatial distribution of the optical properties in the UV and Visible in an aquatic ecosystem.

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 coeff~cientsa nd 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

The optical qualities of shallow wetland lined bays in Lake Victoria

In tropical lakes, the characteristics and dimension of the coastal wetlands can have a strong influence on the quality of the inshore waters. The interaction between littoral wetlands and the open water environment is complex and requires an understanding of the material and energy exchanges between these compartments. In the present analysis, we examine the impact of wetlands on the optical properties (underwater light environment) of two adjacent bays on the Ugandan side of Lake Victoria. We use both irradiance profiles within the water column as well as measurements of the dissolved and particulate fractions of the water column. By introducing a new term to identify that part of the solar spectrum most affected by wetland released dissolved organic matter, it is possible to determine the area of each bay that is influenced by the wetland. Depending on the period of analysis, wetland released chromophoric dissolved organic matter can play a dominating role in the attenuation of UV and visible radiation in the underwater environment. © 2007 Springer Science+Business Media B.V

Remote sensing imagery analysis of the lacustrine system of Iberá wetland

The little-known lacustrine ecosystem of Ibera wetland was examined using satellite-measured reflectance to determine several limnological variables (Secchi depth, Sd; nephelometric turbidity, Tn; dissolved organic matter, DOM). The spatial and temporal analysis has shown the main relationships between the local forcing factors and the ecological state of the aquatic ecosystems. The spatial geomorphologic characteristics of the wetland were strongly related to the spatial distribution of the limnological variables. According to this, the macrosystem has been classified into three large regions that enclose lakes that share diverse characteristics. (i) Northwestern region encloses few open water areas, mainly of small size. (ii) Rounded large lakes (probably originated as oxbow floodplain lakes) are characteristics of the Northeastern region. (iii) Elongated large lakes (probably originated as levee floodplain lakes) are characteristics of the Southern region. These lakes showed the highest Sd and DOM values and the lowest Tn. The different water drainage of the regions seems to be the main cause of the limnological differences of the wetland. The described spatial classification agrees also with other ecological characteristics such as the degree of macrophytes development or composition of the fish community. Different disturbed local areas, overlapping the regional classification, were also identified in the borders of the wetland. These areas have been related with the presence of anthropogenic activities, revealing the sensitivity of these shallow lakes. The whole system showed a relatively homogenous seasonal pattern of water transparency related with solar irradiance cycle. The variability of active floodplains lakes of the Upper Paraná was found to be more strongly influenced by the rainfall, presenting a more unpredictable behaviour