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On benthic fluxes of phosphorus in the Baltic Sea proper – drivers and estimates
Stockholm University, Faculty of Science, Department of Systems Ecology.
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This Thesis focuses on the exchange of phosphorus (P) across the sediment–water interface in the Baltic Sea proper, with particular attention to the influence of bioturbating macrofauna and benthic redox conditions. Benthic P fluxes have major influence on P availability in the water column, which in turn regulates growth conditions for dinitrogen fixating cyanobacteria in the Baltic proper. Presently, a very large area of bottom sediment is overlain by oxygen depleted bottom water and is therefore devoid of aerobic organisms.

In paper I, anoxic sediment from the Western Gotland Basin was oxygenated and exposed to bioturbation by three macrofauna species in a laboratory experiment. The experimental design allowed for detailed studies of how bioturbating animals influence the P fluxes on a species-specific level. All species (Monoporeia affinis, Mysis mixta, and Macoma balthica) mobilised dissolved organic P from the bottom sediment to the supernatant water. Also, particulate P was released by the two former species. None of these P fractions showed any mobility in control sections of the aquarium system. These animal-dependent P fluxes are a previously largely overlooked but potentially significant source of bioavailable P in coastal marine areas, such as the Baltic Sea.

In paper II, we estimate a contemporary reflux of 146 kton dissolved inorganic P (DIP) from bottom sediments in the Baltic proper. This estimate is based on data from a large number of in situ benthic flux measurements using benthic chamber landers along a depth gradient in the Eastern Gotland Basin. DIP effluxes increased with increasing water depth, and decreasing bottom water oxygen concentrations. Bottom water anoxia was identified as a major driver for the mobilisation of DIP from bottom sediments. During such conditions, the DIP efflux was well correlated to carbon oxidation rate, while on oxic bottoms DIP fluxes were low irrespectively of the carbon oxidation rate. Our data support the hypothesis of a positive feedback loop of self-amplifying eutrophication in the Baltic Sea. Thus, both nutrient emission cuts and active mitigation actions to strengthen sedimentary P sinks are warranted for effective remediation of eutrophication in the Baltic Sea.

Place, publisher, year, edition, pages
Stockholm: Department of Systems Ecology, Stockholm University , 2012. , 19 p.
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
URN: urn:nbn:se:su:diva-83098OAI: oai:DiVA.org:su-83098DiVA: diva2:574011
Presentation
2012-12-17, FB105, Svante Arrhenius väg 21A, Stockholm, 15:00 (English)
Opponent
Supervisors
Available from: 2013-01-08 Created: 2012-12-04 Last updated: 2015-03-16Bibliographically approved
List of papers
1. Recolonisation by macrobenthos mobilises organic phosphorus from reoxidised Baltic Sea sediments
Open this publication in new window or tab >>Recolonisation by macrobenthos mobilises organic phosphorus from reoxidised Baltic Sea sediments
2012 (English)In: Aquatic geochemistry, ISSN 1380-6165, E-ISSN 1573-1421, Vol. 18, no 6, 499-513 p.Article in journal (Refereed) Published
Abstract [en]

In recent decades, eutrophication has increased the extent of hypoxic and anoxic conditions in many coastal marine environments. In such conditions, the nutrient flux across the sediment–water interface is a key process controlling the biogeochemical dynamics, and thereby the level and character of biological production. In some areas, management attempts to drive the ecosystem towards phosphorus (P) limitation, which calls for reliable knowledge on the mechanisms controlling P-cycling. We report a well-controlled laboratory experiment on benthic fluxes of P, when shifting from a state of hypoxic and azoic sediments to oxic and zoic bottom conditions. Adding any of three types of macrobenthic fauna (mysid shrimp, pontoporeid amphipod and tellinid clam) to oxygenated aquarium sections resulted in benthic P fluxes that differed consistently from the azoic control sections. All species caused liberation of dissolved organically bound P (DOP) from the sediment, in contrast to the azoic systems. The shrimp and the amphipod also resuspended the sediment, which resulted in a release of P bound to particles (>0.45 μm). Dissolved inorganic phosphate (DIP) was released during hypoxic conditions, but was taken up after oxygenation, irrespective of the presence or absence of bottom fauna. In the presence of fauna, the uptake of DIP roughly equalled the release of DOP, suggesting that the benthic efflux of DOP following oxygenation and bottom fauna (re)colonisation might be considerable. This is an hitherto overlooked animal-controlled nutrient flux, which is missing from coastal marine P budgets.

Keyword
DOP, P retention, Bioturbation, Monoporeia affinis, Macoma balthica, Mysis mixta
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-83090 (URN)10.1007/s10498-012-9172-5 (DOI)000312069200004 ()
Available from: 2012-12-04 Created: 2012-12-04 Last updated: 2017-12-07Bibliographically approved
2. Phosphorus recycling in sediments of the Central Baltic Sea
Open this publication in new window or tab >>Phosphorus recycling in sediments of the Central Baltic Sea
Show others...
2012 (English)In: Biogeosciences Discussions, ISSN 1810-6277, E-ISSN 1810-6285, Vol. 9, 15459-15500 p.Article in journal (Refereed) Published
Abstract [en]

Benthic fluxes of dissolved inorganic phosphorus (DIP) were measured in situ in the Eastern Gotland Basin (EGB), Central Baltic Sea, using benthic landers. A total of 40 flux measurements on 13 stations at water depths ranging from 30–210 m and under different oxygen regimes were carried out on three cruises during three consecutive years (2008–2010) in August–September. Our study is the first to report in situ DIP fluxes in the Baltic Proper, and it provides the most comprehensive data set of benthic DIP fluxes in the Baltic Proper existing to date. DIP fluxes increased with increasing water depth and with decreasing bottom water oxygen concentration. Average fluxes were calculated for oxic bottom water conditions (−0.003 ± 0.040 mmol m−2 d−1), hypoxic conditions (0.027 ± 0.067 mmol m−2 d−1) and anoxic conditions (0.376 ± 0.214 mmol m−2 d−1). The mean flux on anoxic bottoms was ca. 5–10 times higher than previous estimates based on ex situ measurements, but agreed well with previous flux estimations from changes in the basin water DIP pool. The DIP flux was positively correlated with the organic carbon inventory of sediment and the benthic flux of dissolved inorganic carbon (DIC) on anoxic stations, but these variables were uncorrelated on oxic stations. The positive correlation between DIP and DIC fluxes suggests that the benthic DIP flux on anoxic bottoms in the Baltic Proper is mainly controlled by rates of deposition and degradation of organic matter. The flux from anoxic sediment was very P rich in relation to both C and N, and the average C:P ratio in fluxes on anoxic accumulation bottoms was 69 ± 15, which is well below the Redfield C:P ratio of 106:1. On oxic stations, however, the C:P flux ratio was much higher than the Redfield ratio, consistent with well-known P retention mechanisms associated with iron and bacteria in oxidized sediment. Using a benthic mass balance approach, a burial efficiency of 4% was calculated for the anoxic part of the EGB, which suggests that anoxic Baltic sediments are very efficient in recycling deposited P. Based on the measured fluxes and recent estimates of the areal extent of anoxic and hypoxic bottoms, an internal load of 146 kton yr−1 was calculated. This is 7–12 times higher than recent estimates of the external load and clearly highlights the dominance of anoxic sediments as a P source in the Baltic Sea.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-114985 (URN)10.5194/bgd-9-15459-2012 (DOI)
Available from: 2015-03-16 Created: 2015-03-16 Last updated: 2017-12-04Bibliographically approved

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