Disturbance-mediated species loss has prompted research considering how ecosystem functions are changed when biota is impaired. However, there is still limited empirical evidence from natural environments evaluating the direct and indirect (i.e. via biota) effects of disturbance on ecosystem functioning. Oxygen deficiency is a widespread threat to coastal and estuarine communities. While the negative impacts of hypoxia on benthic communities are well known, few studies have assessed in situ how benthic communities subjected to different degrees of hypoxic stress alter their contribution to ecosystem functioning. We studied changes in sediment ecosystem function (i.e. oxygen and nutrient fluxes across the sediment water-interface) by artificially inducing hypoxia of different durations (0, 3, 7 and 48 days) in a subtidal sandy habitat. Benthic chamber incubations were used for measuring responses in sediment oxygen and nutrient fluxes. Changes in benthic species richness, structure and traits were quantified, while stress-induced behavioral changes were documented by observing bivalve reburial rates. The initial change in faunal behavior was followed by non-linear degradation in benthic parameters (abundance, biomass, bioturbation potential), gradually impairing the structural and functional composition of the benthic community. In terms of ecosystem function, the increasing duration of hypoxia altered sediment oxygen consumption and enhanced sediment effluxes of NH4 + and dissolved Si. Although effluxes of PO4 3− were not altered significantly, changes were observed in sediment PO4 3− sorption capability. The duration of hypoxia (i.e. number of days of stress) explained a minor part of the changes in ecosystem function. Instead, the benthic community and disturbance-driven changes within the benthos explained a larger proportion of the variability in sediment oxygen- and nutrient fluxes. Our results emphasize that the level of stress to the benthic habitat matters, and that the link between biodiversity and ecosystem function is likely to be affected by a range of factors in complex, natural environments.

Internal phosphorus (P) loading leading to accelerated eutrophication of marine systems has made acute the research on P chemistry in sediments. Studies on the P chemistry in lake and marine sediments are commonly based on fractionation, where P compounds are divided into several pools according to their solubility in or reactivity with various chemical solutions. Procedures typically begin with dilute extractants, which remove the most loosely bound P forms, and they proceed stepwise toward stronger extractants, attacking P forms more strongly bound to the solid phase (e.g., Ruttenberg 1992).Solutions may contain anions able to displace phosphate (PO 4 -P) from its adsorption sites through competition, anions able to alter the chemical state of the adsorption surface, or anions able to dissolve P-bearing compounds.A wide variety of extraction procedures for soil and sediment P have been developed, varying with the aim of the study and the P fractions targeted (for reviews, see Boström et al. 1982, Van Eck 1982, Pettersson et al. 1988, Ruban et al. 1999. The P forms commonly separated include soluble and loosely sorbed (labile) PO 4 -P, redox-sensitive iron (Fe)-bound P, and P bound to hydrated oxides of (Al) and nonreducible Fe (surface-bound), calcium (Ca)-bound P (apatite-P), and organic P. The organic P is sometimes divided into refractory and labile fractions. In addition to organic P, the refractory fraction is assumed to contain some inorganic P (e.g., Ruttenberg 1992), possibly as occluded P forms.Extraction procedures are often poorly explained in the literature, and the theoretical basis of the P fractions separated using the procedure often remains unclear. In addition, the procedures are often difficult to follow because the practical work is inaccurately described. For instance, the determination of PO 4 -P may be complicated by interference from extractant-derived ions. In this study, a commercial reference material was fractionated using a procedure developed by Psenner AbstractThe aim of this work was to assess the validity of a phosphorus fractionation procedure, introduced by Psenner and co-workers and modified by Jensen and Thamdrup, in chemical characterization of sediment P. In this procedure, P is separated into 6 pools: loosely bound (and pore water) P, redox-sensitive P (bound to iron and manganese), P bound to oxides of aluminum and nonreducible Fe, calcium-bound P, and mobile and immobile pools of organic P. The procedure was slightly modified further, and every step of the work is described in detail. Reproducibility of the method and variation within the extracts obtained at each step were investigated with a commercial reference material. The validity of the results considered against the theoretical basis of the P fractionation procedure was evaluated in terms of the elemental composition of the separate extracts. The results showed good reproducibility of the method; variation in amounts of the different P forms in the separate extractions was small (coefficient of variation <15%)...

Ecosystem functioning is threatened by an increasing number of anthropogenic stressors, creating a legacy of disturbance that undermines ecosystem resilience. However, few empirical studies have assessed to what extent an ecosystem can tolerate repeated disturbances and sustain its multiple functions. By inducing increasingly recurring hypoxic disturbances to a sedimentary ecosystem, we show that the majority of individual ecosystem functions experience gradual degradation patterns in response to repetitive pulse disturbances. The degradation in overall ecosystem functioning was, however, evident at an earlier stage than for single ecosystem functions and was induced after a short pulse of hypoxia (i.e., three days), which likely reduced ecosystem resistance to further hypoxic perturbations. The increasing number of repeated pulse disturbances gradually moved the system closer to a press response. In addition to the disturbance regime, the changes in benthic trait composition as well as habitat heterogeneity were important for explaining the variability in overall ecosystem functioning. Our results suggest that disturbance-induced responses across multiple ecosystem functions can serve as a warning signal for losses of the adaptive capacity of an ecosystem, and might at an early stage provide information to managers and policy makers when remediation efforts should be initiated.

In the sedimental organic matter of eutrophic continental seas, such as the largest dead zone in the world, the Baltic Sea, bacteria may directly participate in nutrient release by mineralizing organic matter or indirectly by altering the sediment’s ability to retain nutrients. Here, we present a case study of a hypoxic sea, which receives riverine nutrient loading and in which microbe-mediated vicious cycles of nutrients prevail. We showed that bacterial communities changed along the horizontal loading and vertical mineralization gradients in the Gulf of Finland of the Baltic Sea, using multivariate statistics of terminal restriction fragments and sediment chemical, spatial and other properties of the sampling sites. The change was mainly explained by concentrations of organic carbon, nitrogen and phosphorus, which showed strong positive correlation with Flavobacteria, Sphingobacteria, Alphaproteobacteria and Gammaproteobacteria. These bacteria predominated in the most organic-rich coastal surface sediments overlain by oxic bottom water, whereas sulphate-reducing bacteria, particularly the genus Desulfobacula, prevailed in the reduced organic-rich surface sediments in the open sea. They correlated positively with organic nitrogen and phosphorus, as well as manganese oxides. These relationships suggest that the bacterial groups participated in the aerobic and anaerobic degradation of organic matter and contributed to nutrient cycling. The high abundance of sulphate reducers in the surficial sediment layers reflects the persistence of eutrophication-induced hypoxia causing ecosystem-level changes in the Baltic Sea. The sulphate reducers began to decrease below depths of 20 cm, where members of the family Anaerolineaceae (phylum Chloroflexi) increased, possibly taking part in terminal mineralization processes. Our study provides valuable information on how organic loading affects sediment bacterial community compositions, which consequently may maintain active nutrient recycling. This information is needed to improve our understanding on nutrient cycling in shallow seas where the dead zones are continuously spreading worldwide.

The effects of the polychaetes Marenzelleria sp. (Polychaeta, Spionidae), nonindigenous, rapidly increasing species in the Baltic Sea, on benthic nutrient fluxes, denitrification and sediment pore water nutrient concentration were studied in laboratory experiments using a flow-through setup with muddy sediment from coastal regions of the Gulf of Finland. In addition, different forms of sediment phosphorus (P), separated by chemical fractionation, were studied in three sediment layers. At a population density corresponding to about half the highest measured in the northern Baltic Sea, Marenzelleria sp. increased the fluxes of P and ammonium to the water column. No effect could be recorded for denitrification. Since the previously dominant species of the area, Monoporeia affinis, can enhance denitrification and lower the amount of dissolved P in the pore water, the replacement of M. affinis with Marenzelleria spp. may lead to increased P flux to the water column and decreased denitrification, further increasing the ammonium flux to the water column. However, sediment reworking by Marenzelleria spp. also oxidizes the surface sediment in the long run, improving its ability to retain P and support nitrification. Therefore, the impact of Marenzelleria spp. on sediment nutrient release may not be as drastic as the initial reactions seen in our experiments suggest.

The chemical composition and vertical distribution of phosphorus (P) in poorly oxygenated sediments in a continuum extending from the open Baltic Sea towards an organic-rich inner bay were characterized by sequential extraction to examine the potential for release of sediment P. The chemical composition of P was related to chemical and physical characteristics of the sediments and the chemistry of pore water and near-bottom water to better understand the behaviour of P. Sediment P increased towards the inner bay, and the concentration of organic matter appeared to dictate its composition: the dominance of apatite-P turned to dominance of organic P (OP). Sediment P burial and, thus, release from sediment P reserves varied depending on the chemical composition of P. Dissolved species at the sediment-water interface suggested fluctuating redox conditions that affect P binding at short time scale. Redox-sensitive, iron (Fe)-bound P was usually relatively low because of poor oxygen (O 2 ) conditions, which emphasized the role of OP in P release. The results indicate that, over the long term, the abundant organic P reserve may support a significant continuing P release from hypoxic sediments in the severely eutrophied Gulf of Finland (GoF) because capture of P into Fe oxyhydroxides at the sediment surface is restricted. The average long-term minimum annual rate of P release from poorly oxygenated sediments below about 60 m depth in the GoF was approximated on the basis of the vertical distribution of sedimentary P forms and estimates of sedimentation rate.

The vertical distribution of various phosphorus (P) forms and their relation to physico-chemical properties of estuary sediment material were studied to better understand the potential release and burial of P. Core samples were taken from two dissimilar estuaries in the Baltic Sea: one in the Archipelago Sea (AS) and one in the Gulf of Finland (GoF). The P reserves were characterized by a sequential extraction procedure including the analysis of simultaneously dissolved elements in two extraction steps. The sediment material was also analysed for particle size distribution and total elements. In addition, several environmental variables were determined. The occurrence of the various forms of P varied with sediment depth among diVerent sites. Reductant soluble, iron (Fe) bound P was the most dynamically changing P form in the sediment, while P bound to other metal oxides and apatite-P were the most stable fractions. High sedimentation rate was a dominating factor for sediment P burial. In addition, the content of organic matter, the amount of erosiontransported sorption components, and the oxygen (O 2 ) conditions in the near-bottom water were important determinants of the behaviour of sediment P. The results indicate that, over the long term, both estuaries have acted as sinks for deposited P and restricted the transport of P to the AS and the open GoF, thereby partly alleviating the eutrophication process.

Whether sediments act as sinks or sources of nutrients depends partly on the oxygen conditions at the seafloor. Laboratory experiments on coastal Gulf of Finland (Baltic Sea) sediment tested the sensitivity of denitrification to a 2 wk anoxia exposure and subsequent reoxidation of the bottom waters. At the same time we followed the rapidly (1 d) and more slowly (9 d) emerging changes in different forms of sediment P after oxic conditions were restored. The total denitrification rate (Dtot) did not change during anoxic incubation, but shifted from coupled nitrification-denitrification (Dn) towards water column nitrate dependence (Dw). As the Dn rate did not decrease at the same rate as the Dw rate increased, the overall effect of 2 wk exposure to anoxia was an increase in Dtot rate. Nitrification was enhanced in the manipulated sediment compared to natural conditions, despite anoxia. Anoxia quickly caused a release of dissolved P from its 2 most labile forms. The effect was readily reversible, but in nature the replenishment of oxygen stores is usually linked to an intense mixing of the water column, and it is possible that part of the P released during anoxia reaches the productive layer. In our experiments, anoxia affected P cycling more than N cycling.