41 research outputs found
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