Coastal and shelf sediments affected by transient or long-term bottom water anoxia and sulfidic conditions undergo drastic changes in macrofauna communities and abundances. This study investigates how early colonization by two macrofaunal functional traits (epifauna vs. infauna) affects oxygen, sulfide, and pH dynamics in anoxic sediment upon recent bottom water oxygenation. Large mesocosms (area 900 cm(2)) with 150-m-deep Baltic Sea soft sediments were exposed to three treatments: (1) no animals; (2) addition of 170 polychaetes (Marenzelleria arctia); (3) addition of 181 amphipods (Monoporeia affinis). Porewater chemistry was investigated repeatedly by microsensor profiling over a period of 65 days. Colonization by macrofauna did not significantly deepen penetration of oxygen compared to the animal-free sediment. Bioturbation by M. affinis increased the volume of the oxidized, sulfide-free sediment by 66% compared to the animal-free control already after 13 days of incubation. By the end of the experiment M. affinis and M. arctia increased the oxidized sediment volume by 87 and 35%, respectively. Higher efficiency of epifaunal amphipods in removing hydrogen sulfide than deep-burrowing polychaetes is likely due to more substantial re-oxidation of manganese and/or nitrogen compounds associated with amphipod mixing activity. Our results thus indicate that early colonization of different functional groups might have important implications for the later colonization by benthic macrofauna, meiofauna and microbial communities that benefit from oxidized and sulfide-free sediments.

Benthic nutrient dynamics in the coastal basin Kanholmsfjarden, NW Baltic proper, were studied by in situ flux measurements and sediment samplings in 2010-2013. The benthic release of NH4 and DIP from anoxic sediments in Kanholmsfjarden were calculated to renew the standing stock inventories of DIN and DIP in the overlying water in roughly 1 year. Starting in summer 2012, mixing of oxygen-rich water into the deep part of the basin temporarily improved the oxygen conditions in the deep water. During the 1 year oxygenated period, the total phosphorus inventory in the surficial sediment increased by 0.4 g P m(-2) or 65%. This was most likely due to stimulated bacterial P assimilation under oxygenated conditions. By July 2013, the bottom water had again turned anoxic, and DIP and DSi fluxes were even higher than earlier in the study period. These high fluxes are attributed to degradation of sedimentary pools of P and Si that had accumulated during the bottom water oxygenation in 2012. The strong correlation between DIP and DSi fluxes and the similar dynamics of DIP and DSi in the sediment pore water and near bottom water, suggest a similar redox dependency of benthic-pelagic exchange for these nutrients.

Effects of bottom water oxygenation and macrofaunal colonisation on benthic fluxes of nitrogen (N), phosphorus (P) and silicon (Si) from long-term anoxic Baltic Sea bottom sediment were investigated. Sediment boxcosms from an anoxic site at 150 m depth in the open Baltic proper were incubated in the laboratory to follow the development of benthic nutrient fluxes during 74 d exposure to flow-through of oxygen-rich water. In contrast to traditional end-point experimental designs, our repeated measurement approach allowed for separation of transient and long-term effects of oxygenation and bioturbation on benthic nutrient recycling. The composition, but not the rate, of the benthic total dissolved N efflux changed by oxygenation from being dominated by NH4 in situ to being mostly composed of NO2 + NO3 and dissolved organic N (DON) under oxic conditions. Oxygenation in the boxcosms decreased the benthic efflux of dissolved silicate (DSi) and essentially shut off the in situ flux of dissolved inorganic phosphorus (DIP). After 20 d of oxygenation, 2 bottom macrofauna taxa, the polychaete Marenzelleria spp. and the amphipod Monoporiea affinis, were introduced to a subset of the boxcosms. Bioturbation by either taxa increased the efflux of dissolved inorganic N (DIN), DON and DSi to the overlying water. The P-rich benthic flux under in situ anoxic conditions roughly approached Redfield N: P stoichiometry after oxygenation in the sediment boxcosms. Upon addition of macrofauna, bioturbation gene rated even higher N:P flux ratios.

This thesis investigates how benthic fluxes of phosphorus (P), nitrogen (N), and silicon (Si) change upon oxygenation of anoxic soft bottoms in the brackish, eutrophicated Baltic Sea. Direct measurements in situ by benthic landers demonstrated that fluxes of dissolved inorganic P (DIP) from anoxic bottom sediments in the Eastern Gotland Basin are higher than previously thought (Paper I). It is argued that the benthic DIP flux has a much larger influence on the DIP inventory in the Baltic proper than the external sources. Similarly, benthic fluxes of DIP and dissolved inorganic N (DIN) from anoxic sediment in the coastal Kanholmsfjärden Basin, Stockholm archipelago, were sufficiently high to renew the pools of these nutrients below the upper mixed layer in roughly one year (Paper II).

A natural inflow of oxygen rich water into the deep, and previously long-term anoxic part of Kanholmsfjärden Basin, increased the P content in the sediment by 65% and lowered DIP and dissolved silica (DSi) concentrations in the pore water. These changes, as well as the large increases in benthic effluxes of these solutes following de-oxygenation of the bottom water, suggest that they are influenced similarly by changing oxygen conditions.

Experimental results in papers III and IV show that common benthic macrofauna species in the Baltic Sea can stimulate benthic release of DIN and DSi, as well as dissolved organic and particulate bound nutrients. Thus, if benthic oxygen conditions would improve in the Baltic, initial effects on benthic–pelagic nutrient coupling will change due to animal colonisation of currently azoic soft bottoms.

A new box corer was designed (Paper V) which can be used to obtain highly needed virtually undisturbed samples from soft bottom sediments – if lowered slowly and straight into the bottom strata – as demonstrated by in situ videography and turbidimetry. The commonly used USNEL box corer caused severe biasing during sediment collection.

An improved, large double-jawed box corer, intended primarily for sampling of soft sediments on continental margins and in large lakes, is described. The device performs reliably when entering the sediment and enclosing the sample, during withdrawal and hoisting on board the ship and also when detaching the collected sediment sample. It offers the following advantages: (1) robust design, (2) minimally impeded flow of water through the box chamber during lowering and (3) an efficient closure mechanism. It is furnished with a supporting stand, a transparent liner and an accessory anti-slosh baffle for insertion in the liner as the corer is set down on the ship's deck. In situ video records and turbidity measurements from field trials, as well as visual inspection of the core and supernatant water after retrieval, show that the device collects minimally disturbed sediment when properly and carefully operated. This contrasts with the bulky United States Naval Electronics Laboratory (USNEL) Spade Corer in which water flow through the box chamber during lowering is impeded, causing a bow-wave ahead of the corer that displaces surficial sediment. In addition, the USNEL's single-spade closing mechanism deforms the sediment sample severely and can even cause loss of sediment. Our new box corer performs much better, making it suitable for quantitative benthic sampling.

Benthic fluxes of phosphorus (P) were measured in situ in the Eastern Gotland Basin (EGB), central Baltic Sea, using benthic landers. A total of 40 flux measurements of dissolved inorganic P (DIP) on 13 stations at water depths ranging 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 dataset of benthic fluxes of DIP and dissolved organic P (DOP) in the Baltic proper existing to date. DIP fluxes increased with increasing water depth and with decreasing bottom water oxygen concentration. Average DIP 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 DIP flux at anoxic bottoms was higher than previous estimates based on ex situ measurements of pore water gradients. The DIP flux was positively correlated with the organic carbon inventory of sediment, and the benthic flux of dissolved inorganic carbon (DIC) at anoxic stations, but these variables were uncorrelated at oxic stations. The positive correlation between DIP and DIC fluxes suggests that the benthic DIP efflux from 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 nitrogen (N). The average C : P ratio in fluxes at anoxic accumulation bottoms was 69+/-15, which is well below the Redfield C : P ratio of 106 : 1. At 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 oxidised sediment. Using a benthic mass balance approach, a burial efficiency estimate of 0.2-12% 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 the average areal extent of anoxic bottoms during years 1999-2006, an internal DIP load of 152 k ton yr(-1) was calculated. This is almost 9 times higher than the average external total phosphorus (TP) supply to the Baltic proper during the same period. This comparison clearly highlights the dominance of internally regenerated P as a DIP source in the Baltic Sea.

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.

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.

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.