Jukka Seppälä

AERONET-OC: a Network for the Validation of Ocean Color Primary Products

Abstract The ocean color component of the Aerosol Robotic Network (AERONET-OC) has been implemented to support long-term satellite ocean color investigations through cross-site consistent and accurate measurements collected by autonomous radiometer systems deployed on offshore fixed platforms. The AERONET-OC data products are the normalized water-leaving radiances determined at various center wavelengths in the visible and near-infrared spectral regions. These data complement atmospheric AERONET aerosol products, such as optical thickness, size distribution, single scattering albedo, and phase function. This work describes in detail this new AERONET component and its specific elements including measurement method, instrument calibration, processing scheme, quality assurance, uncertainties, data archive, and products accessibility. Additionally, the atmospheric and bio-optical features of the sites currently included in AERONET-OC are briefly summarized. After illustrating the application of AERONET-OC dat...

Inducing autoflocculation in the diatom Phaeodactylum tricornutum through CO2 regulation

The effect of pH on flocculation was studied using the diatom Phaeodactylum tricornutum and the green algae Scenedesmus cf. obliquus as surrogate species. There was a distinct, species-specific threshold of pH where flocculation started. P. tricornutum started to flocculate at pHxa010.5 and S. cf. obliquus at pHxa011.3. Above this threshold, settling rates up to 360xa0cmxa0h−1 were observed for P. tricornutum and the concentrating factor was up to 60-fold. The combined effect of pH, turbulence, and cell density on flocculation of P. tricornutum was additionally studied in a factorial 53-design experiment. pH was the most important factor affecting flocculation, but at the pH threshold (pHxa010.5), the concentrating factor was increased by increasing cell density and turbulence. Algae increases the pH during photosynthesis, and the P. tricornutum and S. cf. obliquus cultures increased the pH to a maximum of 10.8 and 9.5, respectively, after discontinuing the CO2 supply. For P. tricornutum, this was above the flocculation threshold, and rapid settling of this species due to increased pH was observed in a matter of hours after the CO2 supply was turned off. This could be used as a simple, low-cost, initial dewatering step for this species.

Nutrient pools, transformations, ratios, and limitation in the Gulf of Riga, the Baltic Sea, during four successional stages

Abstract Basin-wide spatial distribution of different nutrient (N, P) fractions, chlorophyll a and basic hydrography in the Gulf of Riga was studied on five cruises during four stages of the annual succession, in 1993–1995. On the basis of the spatial distributions of the nutrient fractions, a general hypothesis was depicted of the annual transformations in nutrient pools, and their consequences for the nutrient limitation of planktonic production. Total nutrient pools indicate a degree of eutrophication comparable to the middle and eastern Gulf of Finland. In contrast to previous evidence, the Gulf of Riga seems not to be P-limited during the productive season. Our results suggest that the basin is basically N-limited, with hydrographically mediated P-limited phases potentially emerging during the annual succession. In early stages of the productive season, N limitation seems to predominate, with possible P limitation only in the SE parts of the Gulf, affected by the River Daugava spring runoff. During the late summer/early autumn periods when mineralization processes in the water column cumulatively liberate especially ammonium, the depth of the mixed surface layers seems to affect the limitation pattern. Thin or moderate mixed layer favors N limitation and deep mixing favors P limitation, the conditions thus resembling those previously observed in the estuarine eastern Gulf of Finland. With deep mixing, it seems also likely that phytoplankton is not able to fully utilize available nutrients due to the shallow euphotic zone. Therefore, strongly heterotrophic late summer/early autumn stages are likely to emerge. As the simple and exposed topography of the basin makes it exceptionally prone to physical forcing, it is obvious that episodic mixing events can cause considerable interannual and within-season variability in the production preconditions and nutrient limitation of the planktonic community, especially towards late summer and early autumn.

Functional group richness: implications of biodiversity for light use and lipid yield in microalgae.

Currently, very few studies address the relationship between diversity and biomass/lipid production in primary producer communities for biofuel production. Basic studies on the growth of microalgal communities, however, provide evidence of a positive relationship between diversity and biomass production. Recent studies have also shown that positive diversity–productivity relationships are related to an increase in the efficiency of light use by diverse microalgal communities. Here, we hypothesize that there is a relationship between diversity, light use, and microalgal lipid production in phytoplankton communities. Microalgae from all major freshwater algal groups were cultivated in treatments that differed in species richness and functional group richness. Polycultures with high functional group richness showed more efficient light use and higher algal lipid content with increasing species richness. There was a clear correlation between light use and lipid production in functionally diverse communities. Hence, a powerful and cost‐effective way to improve biofuel production might be accomplished by incorporating diversity related, resource‐use‐dynamics into algal biomass production.

Experimental evaluation of nutrient limitation of phytoplankton communities in the Gulf of Riga

Abstract Phytoplankton nutrient limitation was studied in the Gulf of Riga during spring bloom (April 1995), early summer stage (June 1994), cyanobacterial bloom (July 1994) and post cyanobacterial bloom (August 1993). Each year six factorial nutrient enrichment experiments were carried out in various locations in the Gulf; including outer Irbe Strait, northern Gulf and southern Gulf. The responses of natural phytoplankton communities to the nutrient additions (80 μg NH4–N l−1, 20 μg PO4–P l−1 and two levels of combined additions) were followed for 3 days using 6 l experimental units. To evaluate the nutrient limitation patterns, time series of chlorophyll a were analysed using polynomial regression models and ranking method, taking advantage of the relatively constant experimental error. Apparent nutrient depletion rates and ratios were estimated, and compared with the changes in particulate nutrient ratios. During the spring diatom bloom in 1995, ambient inorganic nutrient concentrations were still high, and thus phytoplankton biomass did not respond to additions of nutrients. Chlorophyll a specific nutrient depletion rates were low (0.01–0.12 μg N (μg chl a)−1 h−1 and 0.002–0.016 μg P (μg chl a)−1 h−1) and linear over time, thus also revealing that phytoplankton was not limited by these nutrients in that time. In June 1994, there was an areal shift from N limitation in the outer Irbe Strait towards co-limitation in the southern Gulf. Later in July 1994, during the bloom of N-fixing Aphanizomenon flos-aquae, the N limitation was obvious for the whole study area. For this period chlorophyll a specific nutrient depletion rates were high (0.36–0.67 μg N (μg chl a)−1 h−1 and 0.089–0.135 μg P (μg chl a)−1 h−1), and added nutrients were almost totally depleted during the first light period. After the collapse of cyanobacterial bloom in August 1993, the experiment carried out in the southern Gulf indicated P limitation of phytoplankton. The central Gulf was obviously co-limited, while the area between northern Gulf and outer Irbe Strait was N-limited. Our results indicate that phytoplankton in the Gulf of Riga, earlier considered strictly as P-limited, is at least until late-summer period N- or co-limited. It seems also obvious that there exists a spatial tendency in the phytoplankton limitation patterns, generally from more P- or co-limited southern Gulf towards more N-limited northern basin.

Spatial distribution of phytoplankton in the Gulf of Riga during spring and summer stages

Distribution patterns of chlorophyll a, phytoplankton species and biomass were studied in the Gulf of Riga, one of the most eutrophicated areas of the Baltic Sea. Quasi-synoptic measurements were carried out during four seasonal stages (spring bloom 1995, early-summer stage 1994, cyanobacterial bloom 1994, and late summer stage 1993). For each stage, common factor analysis was used to simplify the highly correlated patterns of nutrients, salinity, temperature and the depth of mixed layer. Obtained latent variables were used to explain spatial distribution of phytoplankton. Generally, the distribution of phytoplankton variables followed closely the patterns of nutrient rich fresh water. During the spring bloom in 1995, the spatial structures of phytoplankton biomass (4–27 mg l−1) and chlorophyll a (13–50 μg l−1) were well explained (r=0.68 and 0.69, respectively, p<0.01) by common factor loaded by inorganic N, total N and P, salinity and the depth of mixed layer. The early-summer stage in 1994 was characterized by homogenous distribution of algal biomass and low concentrations of inorganic N and P. At this stage, the decreasing amount of total N and P from the southern Gulf of Riga towards the northern part of the Gulf was accompanied with the decrease of chlorophyll a concentrations and the increase of relative importance of picoplankton (algae <2 μm). Intense bloom of cyanobacteria (Aphanizomenon flos-aquae (L.) Ralfs) took place in 1994 after warming up of the surface layer. The abundance of A. flos-aquae was related to the common structure of N, P and fresh water and the bloom was most dense (up to 6.6 mg l−1) in the southern Gulf. At the same time, the contribution of picoplankton from total chlorophyll a was high (up to 60%) in the northern Gulf affected less by the nutrient load. The late summer phase in 1993 was characterized by high heterotrophic activities and the phytoplankton community was dominated by cryptomonads (on the average 50% of total biomass). At that stage the distribution of phytoplankton was obviously governed by biological interactions and the multivariate methods were not especially successful to explain the spatial distribution of the main components of phytoplankton community. Excluding the late summer phase, the statistical methods used in this study revealed well the relationships between phytoplankton variables and nutrient rich fresh water. It is evident that nutrient load from the River Daugava is a reason for higher phytoplankton biomass in the southern Gulf of Riga, especially during bloom occasions.

Optimization of variable fluorescence measurements of phytoplankton communities with cyanobacteria

Excitation–emission fluorescence matrices of phytoplankton communities were simulated from laboratory-grown algae and cyanobacteria cultures, to define the optical configurations of theoretical fluorometers that either minimize or maximize the representation of these phytoplankton groups in community variable fluorescence measurements. Excitation sources that match the photosystem II (PSII) action spectrum of cyanobacteria do not necessarily lead to equal representation of cyanobacteria in community fluorescence. In communities with an equal share of algae and cyanobacteria, inducible PSII fluorescence in algae can be retrieved from community fluorescence under blue excitation (450–470xa0nm) with high accuracy (R2xa0=xa01.00). The highest correlation between community and cyanobacterial variable fluorescence is obtained under orange-red excitation in the 590–650xa0nm range (R2xa0=xa00.54). Gaussian band decomposition reveals that in the presence of cyanobacteria, the emission detection slit must be narrow (up to 10xa0nm) and centred on PSII chlorophyll-a emission (~683xa0nm) to avoid severe dampening of the signal by weakly variable phycobilisomal fluorescence and non-variable photosystem I fluorescence. When these optimizations of the optical configuration of the fluorometer are followed, both cyanobacterial and algal cultures in nutrient replete exponential growth exhibit values of the maximum quantum yield of charge separation in PSII in the range of 0.65–0.7.

Phytoplankton pigments and dissolved organic matter distribution in the Gulf of Riga

Abstract The results of field studies of phytoplankton and dissolved organic matter (DOM) in the Gulf of Riga in August–September 1993, June 1994 and April 1995 are presented. Actively excited fluorescence in UV- and visible spectral regions was used to investigate spatial distributions of DOM, Chl a, carotenoids and phycoerythrin in surface water. The fluorescent data were collected by means of laser remote sensing spectrometer (LIDAR), spectrofluorometers in underway flow-through mode and in the measurements of the water samples. Pronounced variable spatial structures of phytoplankton were observed in spring 1995 and late summer 1993, while in early summer 1994 the pigment distribution was rather homogeneous. The spatial modulation of high-resolution profiles of phytoplankton with the period 3.5–5 km was detected across the gulf in all seasons probably caused by variability of hydrophysical conditions. A negative correlation of DOM concentration in surface waters and salinity was revealed in August–September 1993 and June 1994.

On the creation and maintenance of stratification in the Gulf of Riga

Abstract Factors affecting the creation and maintenance of stratification under simple conditions in a boreal estuary (Gulf of Riga, Baltic Sea) are studied in terms of buoyancy forcing. It is found that the ratio r= βC p s α determines if the stratification is created by freshwater or heat fluxes. Here β and α are the haline contraction and thermal expansion coefficients of sea water, s is salinity and C p is specific heat. It is discovered that stratification in this gulf, as well as in the Baltic Sea in general, is created by dilution of salinity due to freshwater fluxes, the thermal contribution being negligible. Data from four cruises in three consecutive years are used to illustrate this effect. An almost ideal river plume bulge and an associated coastal current is found in the gulf, and their spreading rate compared with recent eddy parameterizations. A restratification time scale for the gulf is estimated as 5 weeks and found to be in reasonable agreement with available observations.

Interaction Effects of Light, Temperature and Nutrient Limitations (N, P and Si) on Growth, Stoichiometry and Photosynthetic Parameters of the Cold-Water Diatom Chaetoceros wighamii

Light (20-450 μmol photons m-2 s-1), temperature (3-11°C) and inorganic nutrient composition (nutrient replete and N, P and Si limitation) were manipulated to study their combined influence on growth, stoichiometry (C:N:P:Chl a) and primary production of the cold water diatom Chaetoceros wighamii. During exponential growth, the maximum growth rate (~0.8 d-1) was observed at high temperture and light; at 3°C the growth rate was ~30% lower under similar light conditions. The interaction effect of light and temperature were clearly visible from growth and cellular stoichiometry. The average C:N:P molar ratio was 80:13:1 during exponential growth, but the range, due to different light acclimation, was widest at the lowest temperature, reaching very low C:P (~50) and N:P ratios (~8) at low light and temperature. The C:Chl a ratio had also a wider range at the lowest temperature during exponential growth, ranging 16-48 (weight ratio) at 3°C compared with 17-33 at 11°C. During exponential growth, there was no clear trend in the Chl a normalized, initial slope (α*) of the photosynthesis-irradiance (PE) curve, but the maximum photosynthetic production (Pm) was highest for cultures acclimated to the highest light and temperature. During the stationary growth phase, the stoichiometric relationship depended on the limiting nutrient, but with generally increasing C:N:P ratio. The average photosynthetic quotient (PQ) during exponential growth was 1.26 but decreased to <1 under nutrient and light limitation, probably due to photorespiration. The results clearly demonstrate that there are interaction effects between light, temperature and nutrient limitation, and the data suggests greater variability of key parameters at low temperature. Understanding these dynamics will be important for improving models of aquatic primary production and biogeochemical cycles in a warming climate.