Kai Sørensen

The Response of Experimental Rocky Shore Communities to Nutrient Additions

The aim of this study was to determine whether the experimental nutrient enrichment of littoral rocky shore communities would be followed by a predicted accumulation of fast-growing opportunistic algae and a subsequent loss of perennial benthic vegetation. Inorganic nitrogen (N) and potassium (P) was added to eight concrete mesocosms inhabited by established littoral communities dominated by fucoids. The response to nutrient enrichment was followed for almost 2 1/2 years. Fast-growing opportunistic algae (periphyton and ephemeral green algae) grew significantly faster in response to nutrient enrichment, but the growth of red filamentous algae and large perennial brown algae was unaffected. However, these changes were not followed by comparable changes in the biomass and composition of the macroalgae. The biomass of opportunistic algae was stimulated only marginally by the nutrient enrichment, and perennial brown algae (fucoids) remained dominant in the mesocosm regardless of nutrient treatment level. Established rocky shore communities thus seem able to resist the effects of heavy nutrient loading. We found that the combined effects of the heavy competition for space and light imposed by canopy-forming algae, preferential grazing on opportunistic algae by herbivores, and physical disturbance, succeeded by a marked export of detached opportunistic algae, prevented the fast-growing algae from becoming dominant. However, recruitment studies showed that the opportunistic algae would become dominant when free space was available under conditions of high nutrient loading and low grazing pressure. These results show that established communities of perennial algae and associated fauna in rocky shore environments can prevent or delay the accumulation of bloom-forming opportunistic algae and that the replacement of long-lived macroalgae by opportunistic species at high nutrient loading may be a slow process. Nutrient enrichment may not, in itself, be enough to stimulate structural changes in rocky shore communities.

Validation of MERIS water products and bio-optical relationships in the Skagerrak

The MERIS Level 2 Reduced Resolution products for Case 2 water available in September 2003, and conversion functions used in the reference model, have been validated against in situ data from the Skagerrak, collected during the summers of 2002 and 2003. The MERIS water‐leaving reflectance deviated less than 20% from the measured reflectances in the blue‐green part of the spectrum, but it had a tendency of being overestimated by up to 40% in the blue part of the spectrum and underestimated by up to 20% in the red part. The average relative deviation between the MERIS product for total suspended material (dry weight) and the in situ values was 30%. The MERIS values for chlorophyll a were on an average a factor 2 higher than the in situ values, and a new conversion factor should be used for the Skagerrak area. The absorption coefficients for the sum of yellow substance and bleached particles at 442 nm were underestimated by a factor of up to 10 by the MERIS product. The mean values of the spectral slopes of particle scattering and bleached particle absorption were close to the values of the reference model, while the observed slope of yellow substance was slightly lower than the model slope.

210Pb and 210Po, manganese and iron cycling across the O2/H2S interface of a permanently anoxic fjord : Framvaren, Norway

Abstract Vertical profiles of dissolved and particulate 210 Po and 210 Pb were measured across the redox transition zone at Station F1 in Framvaren Fjord, Norway. In this fjord, a sharp decrease in pH above the O 2 /H 2 S interface facilitates the aerobic dissolution of MnO 2 . In contrast, Fe(II) concentrations begin to increase only at the O 2 /H 2 S interface depth. Activity profiles reveal that dissolved 210 Po and 210 Pb are sequestered efficiently by particulates in surface waters. As polonium-210 and lead-210 activities descend down into the aerobic manganese reduction (AMR) zone, they are remobilized during the reductive dissolution of the carrier phase oxyhydroxides. Both 210 Po and 210 Pb are highly enriched at the O 2 /H 2 S interface where an active community of microbes, such as anoxygenic phototrophs (e.g., Chromatium , Chlorobium sp.), thrives. The coincident peaks in 210 Po , 210 Pb and microbial biomass suggest a strong biological influence on the behavior of these radionuclides. There is a strong covariance between the vertical distribution of Mn and Pb, indicating that their redox cycling is closely coupled and is likely microbially mediated.

The distribution and biomass of phytoplankton and phototrophic bacteria in Framvaren, a permanently anoxic fjord in Norway

The vertical distribution and biomass of phytoplankton and phototrophic bacteria in the permanently anoxic fjord, Framvaren in southern Norway, are described. The distribution of algal and bacterial pigments was studied at different seasons in the period from May 1980 to February 1985. The standing crop of phytoplankton was low in the upper part of the euphotic zone, but increased near the O2/H2S interface. An algal plate and a dense plate of phototrophic bacteria, measured as chlorophyll fluorescence and scattering, were detected near the interface. These plates of phototrophic micro-organisms were found to be photosynthetically active. Sharp concentration peaks near the interface were also found for the active biomass measured as adenosine triphosphate (ATP).

Assessment of phytoplankton class abundance using absorption spectra and chemometrics

In this study, the feasibility of using the in vitro absorption spectra of phytoplankton samples, for determining the relative abundance of specific classes of the phytoplankton, was investigated. Single species cultures of nine algae, representing six phytoplankton classes, were used. Mixtures of algal cultures were also prepared for analysis. In total, 25 samples were examined. Spectra were recorded in the visible wavelength range, 350-770 nm, and evaluated using chemometric methods. First, a principal component analysis (PCA) model was calculated. The score-plot of the first two components showed separation of the different classes, with mixtures of two species appearing between the clusters of each pure culture. Second, a partial least-squares regression, with several independent variables (PLS2), was calculated. The relative phytoplankton class abundances (as a percentage) were used as the dependent variable (y-axis). The average root mean square error of cross-validation (RMSECV) was 8.6%. Hence, for this particular data set, it was possible to estimate the relative abundance of different classes within the samples. The proposed method requires further development, including evaluation of more species representing more phytoplankton classes, and including non-taxonomic features, such as photoadaptation.

Semi-empirical Algorithm for the Retrieval of Ecology-Relevant Water Constituents in Various Aquatic Environments

An advanced operational semi-empirical algorithm for processing satellite remote sensing data in the visible region is described. Based on the Levenberg-Marquardt multivariate optimization procedure, the algorithm is developed for retrieving major water colour producing agents: chlorophyll-a, suspended minerals and dissolved organics. Two assurance units incorporated by the algorithm are intended to flag pixels with inaccurate atmospheric correction and specific hydro-optical properties not covered by the applied hydro-optical model. The hydro-optical model is a set of spectral cross-sections of absorption and backscattering of the colour producing agents. The combination of the optimization procedure and a replaceable hydro-optical model makes the developed algorithm not specific to a particular satellite sensor or a water body. The algorithm performance efficiency is amply illustrated for SeaWiFS, MODIS and MERIS images over a variety of water bodies.

Spectral backscattering coefficient in coastal waters

Based on 186 field observations in the Oslo Fjord of irradiance and radiance and 105 laboratory measurements of beam attenuation, this analysis demonstrates that the ratio between the backscattering coefficient b pb and the scattering coefficient b p of the particles is wavelength dependent and not a constant value. The mean values of b pb/b p at the different wavelengths are close to Petzolds 0.017–0.019 from the San Diego Harbor at 515 nm. For all wavelengths and stations (630 observations) the mean value of b pb/b p is 0.020, the standard deviation of the dataset is 0.015, half of the ratios are greater than 0.020, more than 10% are greater 0.030 and about 5% greater than 0.040. The numerical magnitude of these ratios indicates that small particles are the cause of the latter deviations from the ‘Petzold case’. An example from the northern border of the Skagerrak shows a spectral shape of b pb that can be approximated by λ−1.54, λ being the wavelength, while b p is roughly proportional to λ−0.41. The spectral shape of b pb/b p becomes ∼λ−1.12. During a dinoflagellate bloom in the inner part of the Oslo Fjord direct measurements of b pb have revealed a slope of b pb∼λ−1.59. The spectral shape of the mean values of b p(λ) in the Oslo Fjord, based on the 105 observations, can be approximated by the function ∼λ−0.67, while the spectral shapes of b pb(λ) and b pb(λ)/b p(λ) display a greater variety of forms.

Sources of uncertainty in assessment of marine phytoplankton communities

Characterisation of phytoplankton communities is important for classification of the ecological status of marine waters. In order to design a monitoring programme, it is important to know what degree of variation in the measurements occur at each level (water body, station and sample), so that resources can be spent in a way that maximise the precision of the measured parameters. Seven European water bodies were sampled to assess the variation in pigment concentrations and population densities attributed to water body, station and sample levels. It was found that the main proportion of the variation between pigment measurements was explained by the variation between stations (12–91% of variation) followed by the variation between water bodies (0–89% of variation). For measurements of population density, the main proportion of the variation between densities of cells recorded was explained by the variation between the taxonomists counting the samples (61%), whilst the main proportion of the variation between numbers of taxa recorded was explained by the variation between water bodies (83%). When the cell density of the nine dominant classes were analysed separately, the main proportion of variation was explained at the water body level for all but two class.

An intercomparison of in vitro chlorophyll a determinations for MERIS level 2 data validation

An investigation of the capability of the MERIS (Medium Resolution Imaging Spectrometer) validation teams to determine chlorophyll a, using the latest measuring protocols and advanced high performance liquid chromatography (HPLC), spectrophotometric and fluorometric method has been performed. Two inter‐comparisons, the NIVACal 1 and 2, were made in 2002 as part of the activities of the validation of the MERIS sensor onboard ENVISAT. Eleven validation teams, represented by 20 laboratories, participated in these inter‐comparisons. The algal pigment, chlorophyll a, was determined by either HPLC, spectrophotometric or fluorometric methods for samples of algal cultures, for natural samples collected off the coast of Norway, and for prepared pigment extracts. For both the HPLC and spectrophotometric results systematic errors were dominating. A few laboratories showed random errors. The coefficients of variation (CV) were determined after elimination of “outliers”. The occurrence of “outliers” were frequent for both intercomparisons. The CV for the determined values of chlorophyll a for algal cultures during NIVACal 1, varied from 10 to 25% for the HPLC and from 5 to 25% for the spectrophotometric determination. The CV for the determined values of chlorophyll a in Case II water for natural samples in NIVACal 2 varied from 10 to 16% for HPLC and from 5 to 20% for the spectrophotometric results. The spectrophotometric results were more consistent between the laboratories. The range for CV was larger for the HPLC‐based on chlorophyll a index for Case I water and ranged from 7% to 40%. An intercomparison based on prepared chlorophyll a extracts showed a CV of 8% to 15% for the HPLC and 5% for the spectrophotometric method. Since the CV for analysing the extracts was lower than for extraction and analysis of regular samples, the extraction procedures contribute significantly to the total variation. The recommendation for MERIS validation from this inter‐comparison clearly show the importance of proper extraction procedures of the pigments and the use of common and agreed protocols.

Parameterization of the inherent optical properties of Murchison Bay, Lake Victoria

Lake Victoria, Africas largest freshwater lake, suffers greatly from negative changes in biomass of species of fish and also from severe eutrophication. The continuing deterioration of Lake Victorias ecological functions has great long-term consequences for the ecosystem benefits it provides to the countries bordering its shores. However, knowledge about temporal and spatial variations of optical properties and how they relate to lake constituents is important for a number of reasons such as remote sensing, modeling of underwater light fields, and long-term monitoring of lake waters. Based on statistical analysis of data from optical measurements taken during half a year of weekly cruises in Murchison Bay, Lake Victoria, we present a three-component model for the absorption and a two-component model for the scattering of light in the UV and the visible regions of the solar spectrum along with tests of their ranges of validity. The three-component input to the model for absorption is the chlorophyll-a (Chl-a), total suspended materials concentrations, and yellow substance absorption, while the two-component input to the model for scattering is the Chl-a concentration and total suspended materials.