In littoral sediments, microphytobenthic (MPB) nitrogen assimilation often exceeds nitrogen removal by denitrification, partly because MPB activity suppresses denitrification. Little is known about the balance between these two processes at sublittoral depths. Benthic pigment composition, light and dark oxygen, and nutrient fluxes (NO 3 Ϫ , NH 4 ϩ , dissolved organic nitrogen (DON), PO , Si(OH) 4 ), as well as denitrification were measured between 1 and 3Ϫ 4 15 m in depth in Gullmar Fjord (Skagerrak) in spring and autumn. The hypothesis was that the assimilation/ denitrification ratio would decrease with depth, along with decreasing MPB activity caused by light limitation. MPB photosynthesis occurred along the entire depth gradient, although sediments were net autotrophic only above 5 m. Inorganic nitrogen (DIN) (and silica) flux changed along the depth gradient, the general pattern being sediment uptake at Յ5 m and efflux at Ն10 m depth. DON flux (ϳ50% of total dissolved nitrogen flux) showed a less clear pattern. Two trends regarding DIN fluxes and denitrification-significant light effects and negative correlations with gross primary productivity-showed that MPB activity influenced nitrogen (N) turnover. Although denitrification increased with depth, rates remained low (Ͻ0.4 mmol N m Ϫ2 d Ϫ1 ), and MPB assimilation (0.2-3.6 mmol N m Ϫ2 d Ϫ1 ) exceeded or equaled denitrification. MPB incorporated ϳ35% of the remineralized N along the depth gradient, whereas denitrification removed ϳ20%. Thus, the influence of MPB on benthic nitrogen turnover, denitrification included, extends to sublittoral depths. Further, denitrification does not necessarily remove more N in the deeper, heterotrophic part of the photic zone, compared to the littoral, autotrophic zone.

It is recognized that microorganisms inhabiting natural sediments significantly mediate the erosive response of the bed (“ecosystem engineers”) through the secretion of naturally adhesive organic material (EPS: extracellular polymeric substances). However, little is known about the individual engineering capability of the main biofilm components (heterotrophic bacteria and autotrophic microalgae) in terms of their individual contribution to the EPS pool and their relative functional contribution to substratum stabilisation. This paper investigates the engineering effects on a non-cohesive test bed as the surface was colonised by natural benthic assemblages (prokaryotic, eukaryotic and mixed cultures) of bacteria and microalgae. MagPI (Magnetic Particle Induction) and CSM (Cohesive Strength Meter) respectively determined the adhesive capacity and the cohesive strength of the culture surface. Stabilisation was significantly higher for the bacterial assemblages (up to a factor of 2) than for axenic microalgal assemblages. The EPS concentration and the EPS composition (carbohydrates and proteins) were both important in determining stabilisation. The peak of engineering effect was significantly greater in the mixed assemblage as compared to the bacterial (x 1.2) and axenic diatom (x 1.7) cultures. The possibility of synergistic effects between the bacterial and algal cultures in terms of stability was examined and rejected although the concentration of EPS did show a synergistic elevation in mixed culture. The rapid development and overall stabilisation potential of the various assemblages was impressive (x 7.5 and ×9.5, for MagPI and CSM, respectively, as compared to controls). We confirmed the important role of heterotrophic bacteria in “biostabilisation” and highlighted the interactions between autotrophic and heterotrophic biofilm consortia. This information contributes to the conceptual understanding of the microbial sediment engineering that represents an important ecosystem function and service in aquatic habitats.

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Natural sediment stability is a product of interacting physical and biological factors, and whereas stability can be measured, few techniques allow sensitive assessment of the sediment surface as conditions change. For example, stability gradually increases as a biofilm develops or as salinity rises, or it might be influenced by toxic compounds. This article introduces a new technique (magnetic particle induction: MagPI) based on the magnetic attraction of specially produced fluorescent ferrous particles. The test particles are added to a surface and subjected to an incrementally increasing magnetic field produced by permanent magnets or electromagnets. There is a strong correlation between magnetic flux density (mTesla) and distance from the surface (r 2 = 0.99) for permanent magnets and between magnetic flux density and the current supplied to an electromagnet (r 2 > 0.95). The magnetic force at which the particles are recaptured is determined as a measure of surface adhesion. MagPI therefore determines the "stickiness" of the surface, whether a biofilm, sediment, or other material. The average magnetic flux density required to remove test particles from diatom biofilms (15.5 mTesla) was significantly greater than from cyanobacterial biofilms (10 mTesla). Controls of fine glass beads showed little adhesion (2.2 mTesla). Surface adhesion is an important bed property reflecting the sediment system's potential to capture and retain new particles and accumulate material. MagPI offers a straightforward and economic way to determine the surface adhesion of a variety of surfaces rapidly and with precision. The technique may have applications in physical, environmental, and biomedical research.

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Effects of nutrient status and short-term, low-level exposure to the antifouling biocide copper pyrithione (CPT) on intact shallow-water sediment were studied in an outdoor experiment, focusing on basic ecosystem functions driven by microorganisms. These functions were trophic status (autotrophy/heterotrophy), direction of dissolved inorganic nitrogen (DIN) flux, and the ratio of algal N assimilation:denitrification, and were based on measurements of daily sediment -water oxygen and nitrogen fluxes and denitrification. Bacterial production and biomass of microphytobenthos, meiofauna, and bacteria were also measured. The recovery of the functions was studied for 38 d. CPT exposure did not change basic ecosystem functions; the sediment remained autotrophic and a net sink of DIN, where microalgal N incorporation far exceeded denitrification. However, CPT affected individual N-cycling variables (ammonium flux, denitrification), as well as community respiration and bacterial production, with the direction of the response depending on sediment nutrient status. Significant CPT effects were more frequent in the nutrient-enriched sediment. Direct toxic effects were followed by indirect food-web-mediated (top-down) effects, leading to different timing and direction of effects with different sediment nutrient status. Permutational multivariate analysis of variance showed that initial CPT effects on system functions in both nutrient regimes were followed by an oscillating recovery process, where effects remained longer in the nutrient-enriched sediment. While single CPT exposure events may not affect general metabolic functions (oxygen flux) -at least not in net autotrophic sediments -repeated CPT exposure of sediment microbiota may impact nitrogen cycling in shallow, eutrophied waters.

Recovery of microphytobentos (MPB) and benthic processes were followed during 23 d after sediment deposition simulating the effects of 1 wk of nearby dredging or construction work. Cores of natural intact sediment in an outdoor flow-through system were exposed to daily depositions of 1.5 mm fine-grained sediment over 7 d (total load 10.5 mm). Porosity, chlorophyll a (chl a; proxy for MPB biomass), denitrification and sediment−water fluxes of oxygen and inorganic nutrients were measured during day and night on 6 occasions. After deposition stopped, chl a in the uppermost 3 mm of the sediment had decreased to 25% of that in the controls, started to increase linearly, probably due to upward migration of diatoms, but did not converge with the control cores in the course of the experiment. The linear increase of chl a indicated a recovery of algal biomass after ~50 d. The proportion of large sigmoid diatoms increased in the deposition cores and this change in MPB composition remained over the 23 d. Deposition resulted in higher porosity and increased flaking of the newly established algal mat. Deposition generally increased release or decreased uptake of nutrients, though effects on nitrate flux and denitrification were less clear. Although alga-related functions (oxygen production and nutrient fluxes in light) recovered faster than algal biomass, the faster recovery of the integrated system function in the dark reflected the impact of deposition on MPB. Sediment deposition in microtidal areas may imply disturbances for MPB, threatening the food supply for grazers and deposit feeders, and, in the end, fish that use the shallow areas as nurseries. KEY WORDS: Microphytobenthos · Sediment deposition · Oxygen flux · Nutrient flux · Denitrification · Resilience Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 446: [31][32][33][34][35][36][37][38][39][40][41][42][43][44] 2012 tute the base of the food webs of shallow-water sediments. Despite this obvious dependency on lower trophic levels there are surprisingly few studies that have considered the effects of sediment deposition on microbenthic populations at the sediment−water interface (Wulff et al. 1997, Schratzberger et al. 2000, let alone functions controlled by them (Rodil et al. 2011).In shallow-water sediments, microbenthic organisms drive central ecosystem processes, such as primary production, decomposition and remineralisation, and therefore play a key role in biogeochemical cycling (e.g. Hochard et al. 2010 and references therein). In photic sediments, the autotrophic component consisting of microalgae and cyanobacteria (microphytobenthos, MPB) often dominates these communities (Paerl & Pinckney 1996). MPB are not only important primary producers (Underwood & Kromkamp 1999, Haese & Pronk 2011, but also control sediment−water nutrient fluxes, often turning sediment into nutrient sinks (Sundbäck et al. 2004 and references therein), and bacterial nitrogen turnover (Risgaard Petersen 2003). They also func...

The complex systems of shallow-water sediments are today subjected to varying nutrient inputs combined with other anthropogenic stressors, such as toxicants. The effects of differences in nutrient status of the sediment combined with short-term, low-level exposure to the anti-fouling biocide copper pyrithione (CPT) were studied in a 38 d experiment using intact sediment in a flowthrough mesocosm system. Abundance and diversity of microphytobenthos, bacteria and meiofauna were assessed together with sediment -water oxygen and nutrient fluxes (day and night) and denitrification. More effects were found for the sediment with higher nutrient status, but these combined effects could not be predicted by adding the separate effects of nutrients and CPT observed for the sediment with low nutrient status. Biomass and diversity of microphytobenthos increased later on in the experiment due to CPT exposure, as did bacterial activity and abundance. CPT exposure also increased the proportion of nematodes in the meiofauna. Generally, prokaryotic functions appeared more affected by CPT exposure than eukaryotic. Differences in the response of functional variables suggest that eukaryotic photoautotrophs possess a higher functional redundancy than heterotrophs. Among nutrient fluxes the nitrogen cycling was affected through changed rates of ammonium flux and denitrification. The integrated analyses show a clearer effect of CPT on the community structure under low-nutrient status, while CPT affected the community function more in the high-nutrient system. All treatments show convergence of function, in both light and dark, towards the end of the experiment, whereas the structure remained separated due to the nutrient regime.KEY WORDS: Combined stressors · Sediment · Eutrophication · Copper pyrithione · Nutrient cycling · Microphytobenthos · Bacteria · Meiofauna Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 48: [277][278][279][280][281][282][283][284][285][286][287][288][289][290][291][292][293][294] 2007 rect effects occur when the stressors are combined compared to when acting alone (Skei et al. 2000, Laursen et al. 2002, Millward et al. 2004. Interactions between stressors in a system might cause non-additive effects, giving a totally different response than what would have been the case assuming simple additive effects (Folt et al. 1999).Cloern (2001) has reviewed and described the conceptual models of coastal eutrophication, which include 3 phases. The first 2 phases are fairly simple, and include only the signal of nutrient enrichment and the responses to enrichment. These 2 are relatively well investigated. In the third phase, other stressorsincluding toxicants -are added in combination with nutrient enrichment. This third phase is at present not well studied, and we hope that the present study can contribute information suitable to represent the third phase in a conceptual model.In the present experiment we studied the combined effects of eutrophication and the a...