Under nutrient-constrained conditions phytoplankton exudates represent a large proportion of the photosynthetically produced organic carbon, because organic carbon production cannot be built into biomass due to lack of e.
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Exudates consist of simple sugars that are a prime carbon source for bacteria, which, in nutrient poor waters become highly competitive for N and P. In oligotrophic aquatic systems the bacteria to phytoplankton biomass ratio is therefore high e. Because large cells have higher uptake capacity than smaller cells, high nutrient concentrations will lead to a dominance of relatively large phytoplankton. Since N and P are available to produce new phytoplankton biomass, a smaller proportion of the primary production is lost via exudation . In nutrient rich waters therefore, the phytoplankton to bacterial biomass ratio is high.
Another important mechanism governing the phytoplankton-bacteria balance is ecosystem CNP stoichiometry .
Eutrophication in Coastal Marine Ecosystems | Coastal and Estuarine Studies
In particular it has been observed that if dissolved organic carbon is provided, a food web may switch from dominance of phytoplankton production to dominance of heterotrophic bacterial production. Climate change has been predicted to cause increased precipitation in northern Europe  , with the consequence of increasing dissolved allochthonous organic carbon inputs into high latitude seas . It is likely that increased runoff of colored dissolved organic matter from the terrestrial system may have especially large effects on ecological function of semi-enclosed marine systems, like the inner Baltic Sea and fjords.
High allochthonous organic carbon inputs may have negative effects on phytoplankton growth via two different mechanisms: 1 light climate deterioration, since inflowing organic matter is often brown, i. Since the food web is size-structured e. Therefore more losses may occur in food webs in low productive than in more productive systems. The food web efficiency, defined as the productivity at higher trophic levels normalized to the productivity at the basal level, would thus be higher in more productive than in unproductive areas.
On the other hand eutrophication in fresh or brackish waters often leads to a dominance of filamentous cyanobacteria  , which are toxic, less edible and of poorer nutrient quality for zooplankton e. Under such conditions food web efficiency may be low. This suggests that food web efficiency shows a hump-shaped relation along nutrient gradients.
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However, darkening of the seawater may also have direct effects on higher trophic levels, for example induce a shift from visual to non-visual predators that could cause declined fish yield  , . To test the effect of allochthonous matter on food web production and efficiency, an experiment was performed with two types of enrichments gradients; one with NP nitrogen and phosphorus and one with CNP carbon, nitrogen and phosphorus.
Seawater from the northern Baltic Sea was used and N and P were added as inorganic nutrients and C was added in the form of humus. We hypothesized that increasing NP load would increase production at all trophic levels, i.
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Furthermore, we hypothesized that food web efficiency would show a hump-shaped relation to NP addition, due to variations in phytoplankton cell sizes and their edibility. At low NP concentration the phytoplankton would be small and the number of tropic levels numerous, whereas at higher NP concentrations the phytoplankton would be larger and the number of trophic levels fewer, however at very high NP concentrations the phytoplankton would be dominated by relatively inedible filamentous forms leading to low food web efficiency.
In the CNP enrichments the food web efficiency would be consistently low due to relatively many trophic levels. The results of the study are used to discuss potential effects of climate change and anthropogenic nutrient load on food web function and productivity in coastal marine systems. Before the start of the experiment we searched for a suitable allochthonous carbon source. To select a representative substance, a bacterial growth experiment was performed where four different commercially available carbon sources were compared to allochthonous carbon isolated from river water entering the northern Baltic Sea.
The water in the bags was enriched with an NP solution corresponding to thirty percent of the winter concentrations in the northern Baltic Sea 1. The humic substance was isolated as dried powder and the molecular weight was approximately Three Whirl-Pak bags were established as controls, where carbon was not added.
Growth rates were determined for each of the three replicates from log-normal growth curves. Average values and standard deviation were calculated for the six treatments. Acetanilide was utilized for standardization. The absorption of photosynthetically active radiation PAR, — nm by natural FO and HF were tested using a scanning spectrophotometer Thermo Fisher, Evolution ; slit width 1 nm in seawater adjusted with humic substances, prior to microbial growth incubation.
Water was stored in four 2. The collection process, including transportation to the laboratory, took less than 2 hours. This process was completed in less than 3 hours. In the NP treatments 10 tanks inorganic nitrogen and phosphorus was added in an eight-step gradient, ranging from 0 to 0.
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The treatments were named 0, 5, 10, 15, 20, 25, 30 and 35 NP, indicating the percentage of the winter N and P concentrations that were added daily. The 10 NP treatment was replicated three times daily addition of 0. In the CNP treatments 9 tanks , a daily addition of inorganic nitrogen and phosphorus was made as described above along with additions of humic carbon HF at The CNP enrichment series comprised the levels 5, 10, 15, 20, 25, 30 and The 10 CNP treatment was replicated three times.
Water samples were collected at regular intervals in the middle of the tanks, using a Ruttner sampler. Samples were collected on days 6, 14, 21, 28 and 35, henceforth referred to as week 1, 2, 3, 4 and 5, respectively. PAR measurements were performed at five spatially distributed positions within each mesocosm at 0, 20, 40 and 50 cm depths, obtaining a total of 20 data points per mesocosm. Area and depth weighted average PAR values were then calculated. Concentrations of dissolved organic carbon DOC and particulate organic carbon POC were analyzed in week 4 and 5 to confirm that enrichment within the mesocosms had taken place.
Approximately 10 ml of water was filtered through a 0. All materials in contact with the samples, including the filters and filter units, were acid washed with 1. Acetanilide was utilized for standardization, and results were corrected for blank filter carbon content. Total organic carbon TOC content was calculated according to:. For analysis of inorganic nutrients, 50 ml of water was filtered through a 0.
For Tot N and Tot P, unfiltered water was stored frozen until analysis. Dissolved inorganic phosphorous DIP was set equal to phosphate concentration and dissolved inorganic nitrogen DIN as the sum of nitrate, nitrite and ammonium. Mesozooplankton were analyzed from 5. A Leica stereomicroscope was used to determine the species composition and abundance of mesozooplankton communities. Since the mesozooplankton community was totally dominated by copepods, other groups were ignored in the calculations.
Lengths were transformed to body mass using the length to weight regressions  , and assuming a carbon content of 5. Ciliate and phytoplankton samples were preserved with 0. Ciliate samples 50 ml were settled in a sedimentation chamber for 24—48 h. Carbon biomasses were calculated according to Menden-Deuer and Lessard .
Eutrophication: Causes, Consequences, and Controls in Aquatic Ecosystems
The size classes were chosen on the basis of the size of the largest dimension of single cells, filaments or colonies. Samples for analysis of heterotrophic bacteria, picocyanobacteria and flagellates were preserved with 0. For analysis of heterotrophic bacteria, 1—3 ml was filtered onto black 0. Cell abundances and biovolumes were analyzed using an epifluorescence microscope Nikon TE using blue excitation light connected to an image analysis system . Carbon biomass was calculated according to Norland . For analysis of flagellates and picocyanobacteria 5—10 ml water was filtered onto black 0.
Flagellates were counted in UV light, scanning one diagonal across the filter. Picocyanobacteria were counted by autofluorescence in green excitation light. Primary production Pp was measured using the 14 C technique . Triplicate light and one dark sample each 9 ml were incubated in acid-washed polycarbonate bottles with After adding scintillation cocktail Optiphase, Hi-Safe 3 , samples were analyzed in a Beckman scintillation counter.
Gross Pp g and net Pp n primary production was calculated according to Gargas . Bacterial production Bp was measured using the [ 3 H]-leucine technique  Smith and Azam Triplicate 1. The samples were rinsed by the addition of 1. Liquid scintillation cocktail 1. Incorporated leucine was measured with a Beckman scintillation counter and bacterial production was calculated following Simon and Azam . Respiration was measured as oxygen consumption. Respiration R was calculated according to Valiela  :.
Respiratory quotient RQ was set to 1, which has been measured for organisms metabolizing carbohydrates. This value was used for the entire plankton community, although the measurements comprised both autotrophic and heterotrophic organisms. Net ecosystem production NEP was calculated according to:.
Samples were collected between late May and early June as this period represented free-flowing water conditions. River samples were collected a few kilometers upstream at one to six locations and on one to three different occasions. Estuarine samples were collected at five or six locations on one to three different occasions.
Total P and N samples were analyzed using standard monitoring methods . No specific permits were required for the performed field studies in this work.
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