Timo Tamminen

Internal ecosystem feedbacks enhance nitrogen-fixing cyanobacteria blooms and complicate management in the Baltic Sea

Abstract Eutrophication of the Baltic Sea has potentially increased the frequency and magnitude of cyanobacteria blooms. Eutrophication leads to increased sedimentation of organic material, increasing the extent of anoxic bottoms and subsequently increasing the internal phosphorus loading. In addition, the hypoxic water volume displays a negative relationship with the total dissolved inorganic nitrogen pool, suggesting greater overall nitrogen removal with increased hypoxia. Enhanced internal loading of phosphorus and the removal of dissolved inorganic nitrogen leads to lower nitrogen to phosphorus ratios, which are one of the main factors promoting nitrogen-fixing cyanobacteria blooms. Because cyanobacteria blooms in the open waters of the Baltic Sea seem to be strongly regulated by internal processes, the effects of external nutrient reductions are scale-dependent. During longer time scales, reductions in external phosphorus load may reduce cyanobacteria blooms; however, on shorter time scales the internal phosphorus loading can counteract external phosphorus reductions. The coupled processes inducing internal loading, nitrogen removal, and the prevalence of nitrogen-fixing cyanobacteria can qualitatively be described as a potentially self-sustaining “vicious circle.” To effectively reduce cyanobacteria blooms and overall signs of eutrophication, reductions in both nitrogen and phosphorus external loads appear essential.

Diversity predicts stability and resource use efficiency in natural phytoplankton communities.

The relationship between species diversity and ecosystem functioning has been debated for decades, especially in relation to the “macroscopic” realm (higher plants and metazoans). Although there is emerging consensus that diversity enhances productivity and stability in communities of higher organisms; however, we still do not know whether these relationships apply also for communities of unicellular organisms, such as phytoplankton, which contribute ≈50% to the global primary production. We show here that phytoplankton resource use, and thus carbon fixation, is directly linked to the diversity of phytoplankton communities. Datasets from freshwater and brackish habitats show that diversity is the best predictor for resource use efficiency of phytoplankton communities across considerable environmental gradients. Furthermore, we show that the diversity requirement for stable ecosystem functioning scales with the nutrient level (total phosphorus), as evidenced by the opposing effects of diversity (negative) and resource level (positive) on the variability of both resource use and community composition. Our analyses of large-scale observational data are consistent with experimental and model studies demonstrating causal effects of microbial diversity on functional properties at the system level. Our findings point at potential linkages between eutrophication and pollution-mediated loss of phytoplankton diversity. Factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.

Decadal-scale changes of dinoflagellates and diatoms in the anomalous baltic sea spring bloom.

The algal spring bloom in the Baltic Sea represents an anomaly from the winter-spring bloom patterns worldwide in terms of frequent and recurring dominance of dinoflagellates over diatoms. Analysis of approximately 3500 spring bloom samples from the Baltic Sea monitoring programs revealed (i) that within the major basins the proportion of dinoflagellates varied from 0.1 (Kattegat) to >0.8 (central Baltic Proper), and (ii) substantial shifts (e.g. from 0.2 to 0.6 in the Gulf of Finland) in the dinoflagellate proportion over four decades. During a recent decade (1995-2004) the proportion of dinoflagellates increased relative to diatoms mostly in the northernmost basins (Gulf of Bothnia, from 0.1 to 0.4) and in the Gulf of Finland, (0.4 to 0.6) which are typically ice-covered areas. We hypothesize that in coastal areas a specific sequence of seasonal events, involving wintertime mixing and resuspension of benthic cysts, followed by proliferation in stratified thin layers under melting ice, favors successful seeding and accumulation of dense dinoflagellate populations over diatoms. This head-start of dinoflagellates by the onset of the spring bloom is decisive for successful competition with the faster growing diatoms. Massive cyst formation and spreading of cyst beds fuel the expanding and ever larger dinoflagellate blooms in the relatively shallow coastal waters. Shifts in the dominant spring bloom algal groups can have significant effects on major elemental fluxes and functioning of the Baltic Sea ecosystem, but also in the vast shelves and estuaries at high latitudes, where ice-associated cold-water dinoflagellates successfully compete with diatoms.

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.

Nutrient runoff and transfer from land and rivers to the Gulf of Riga

In this paper, we have synthesized and integrated results regarding nutrient loads and eutrophication of the Gulf of Riga (GoR) that were obtained in three projects that were part of a six-year research programme (1993–1998). In particular, we focused our attention on the factors that control the temporal variability in the load of nutrients in the drainage basin and rivers, as well as the effects of such nutrients on the environment of the recipient, the Gulf of Riga.The results indicate that the rivers play a crucial role in the total input of nutrients to the GoR, and exceed the combined contribution from atmospheric deposition, point emissions from cities and industries along the coast, and nitrogen fixation by marine organisms. It was found that natural variability in water discharge was the main factor controlling the temporal variability in the riverine load of nutrients (i.e. differences in load between seasons and years). Moderate nutrient losses to surface waters, especially from agriculture, and weak riverine response to the sudden decrease in agricultural production after the disintegration of the Soviet Union in the early 1990s were also found. It is suggested that this is most likely related to the inertia in, and buffering capacity of, agricultural soils, i.e. depending of factors such as hydrological conditions, the size of ground water aquifers and water-saturated soils, creating favourable conditions for nutrient retention processes in the agricultural landscape. Regardless of the relatively low area-specific riverine inputs, the pollution loads in the rivers have contributed significantly to eutrophication of the Gulf of Riga. This seems to be true despite the relatively rapid distribution of the loads beyond the littoral zone of the gulf, resulting in a moderate all-over eutrophication of the entire recipient, characteried by a continuously shifting gradient between the southern and northern parts of the gulf. Furthermore, the marine eutrophication seems to be less prominent and less confined to the southernmost basin than previously assumed.

Specific Affinity for Phosphate Uptake and Specific Alkaline Phosphatase Activity as Diagnostic Tools for Detecting Phosphorus-limited Phytoplankton and Bacteria

We analyzed responses of soluble reactive phosphorus (SRP), bioavailable phosphate (PO4), particulate phosphorus, turnover time of orthophosphate (Tt), and alkaline phosphatase activity (APA) to varying degrees of nutrient stress. The nutrient stress was evoked by different treatments in concentration and combination of inorganic nitrogen (N) and phosphorus (P), and labile organic carbon (glucose) to mesocosms in experiments carried out in eutrophic southern (Odense Fjord, Denmark) and northern (Tvärminne Archipelago, Finland) coastal zones of the Baltic Sea. Despite seasonal and geographical differences, similar responses were observed in both experiments. Low SRP (<100 nmol l−1), shortTt (<10 h), and increased levels of APA were observed in both N+P balanced and P deficient treatments, while the opposite trend was observed in P replete treatments. The shortestTt and the highest APA were found when glucose was combined with N treatment. Bioavailable PO4 was estimated usingTt and P uptake rates as derived from stoichiometric conversion of carbon based primary and bacterial production. With shorterTt, the PO4 pool declined to <1 nmol-P l−1, whereas the SRP background pool (difference between SRP and PO4) remained relatively constant (c. 50 nmol l−1). APA was inversely related to PO4 but not to SRP. Responses of specific APA and specific affinity for PO4 uptake, which are APA and PO4 uptake rates (inverse ofTt), respectively, normalized to the summed P biomass of phytoplankton and bacteria, responded consistently to the pattern and magnitude of nutrient limitation evoked in our experiments. Our results, together with a literature survey, suggest that both parameters can be useful in examining PO4 availability for the natural phytoplankton and bacteria community in P starved aquatic systems.

Comparative analysis of food webs based on flow networks: effects of nutrient supply on structure and function of coastal plankton communities

The objective of COMWEB was to develop efficient analytical, numerical and experimental methods for assessing and predicting the effects of nutrient (N, P, Si) supply on the stability and persistence of pelagic food web structure and function in coastal waters. The experimental comparative work included a geographic gradient covering Baltic, Mediterranean, and NE Atlantic waters and a NE Atlantic gradient in state of eutrophication. COMWEB has been an experimental approach to coastal eutrophication, studying effects of enhanced nutrient supply on components and flows of the entire lower pelagic food web. Flow network representations of pelagic food webs has been a framework of data reduction and flows were established by sophisticated inverse modelling. Fundamental information on physiological properties of functional key species in the pelagic food web was used to constrain flow estimations. A main conclusion derived from the flow networks was that very little energy and materials were transferred from the microbial food web to the main food chain. The lower food web could therefore be described as two parallel food chains with relatively limited interaction between heterotrophic groups. Short-term effects of nutrient perturbations were examined in mesocosms along the geographic gradient. The response was comparable in all systems, with a stronger effect on the activity and biomass of autotrophic groups than those of heterotrophic ones. Mediterranean waters showed much lower autotrophic biomass response than Baltic and NE Atlantic waters, which responded almost equally. The response of primary production was, however, more comparable. High phytoplankton lysis rate explained this low accumulation of biomass in Mediterranean waters. The study of Atlantic coastal waters of different eutrophic states revealed that the ecological response was higher in the closed nutrient perturbed mesocosms than in open systems exposed for >4 summer months (summer/autumn season). The Atlantic lagoon evolved gradually from the natural oligotrophic situation towards the more eutrophicated North Sea during fertilisation. The responses observed on seasonal and long-term scale (>10 years) may therefore be equal. The differences between short-term (weeks) and intermediate-term (seasonal) responses is most likely a result of the different time scales of perturbation and observation and the variable exchange rates with surrounding waters (water dilution rate). The analysis of pelagic flow networks provided a framework of diagnostic criteria for state and quality assessment of coastal waters. The nutrient loading rates related better to estimates of biotic fluxes than to concentrations of biotic compartments and total nutrients. On the contrary, the concentration of biotic compartments, or the biomasses, related better to total nutrient concentrations. Primary production, mesozooplankton grazing and growth, fraction of primary production consumed by grazers, bacterial production relative to primary production, cycling indices, and path lengths were all well related to nutrient loading rate. Autotrophic biomass, ratio of autotrophic to heterotrophic biomass, and fraction of pico-cyanobacteria of total autotrophic biomass were all related to total nutrients. Some of these variables, which responded equally in all systems, have the potential of becoming unified response functions in a management model for European coastal waters. COMWEB has provided further insight into the mechanisms behind coastal eutrophication. A main achievement is the conceptual framework for unified response functions, important components of management models for nutrient emission to coastal waters.

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.