Habitat diversity and type govern potential nitrogen loss by denitrification in coastal sediments and differences in ecosystem-level diversities of disparate N2O reducing communities

In coastal sediments, excess nitrogen is removed primarily by denitrification. However, losses in habitat diversity may reduce the functional diversity of microbial communities that drive this important filter function. We examined how habitat type and habitat diversity affects denitrification and the abundance and diversity of denitrifying and N2O reducing communities in illuminated shallow-water sediments. In a mesocosm experiment, cores from four habitats were incubated in different combinations, representing ecosystems with different habitat diversities. We hypothesized that habitat diversity promotes the diversity of N2O reducing communities and genetic potential for denitrification, thereby influencing denitrification rates. We also hypothesized that this will depend on the identity of the habitats. Habitat diversity positively affected ecosystem-level diversity of clade II N2O reducing communities, however neither clade I nosZ communities nor denitrification activity were affected. The composition of N2O reducing communities was determined by habitat type, and functional gene abundances indicated that silty mud and sandy sediments had higher genetic potentials for denitrification and N2O reduction than cyanobacterial mat and Ruppia maritima meadow sediments. These results indicate that loss of habitat diversity and specific habitats could have negative impacts on denitrification and N2O reduction, which underpin the capacity for nitrogen removal in coastal ecosystems

Toxic Algae Silence Physiological Responses to Multiple Climate Drivers in a Tropical Marine Food Chain

Research on the effects of climate change in the marine environment continues to accelerate, yet we know little about the effects of multiple climate drivers in more complex, ecologically relevant settings – especially in sub-tropical and tropical systems. In marine ecosystems, climate change (warming and freshening from land run-off) will increase water column stratification which is favorable for toxin producing dinoflagellates. This can increase the prevalence of toxic microalgal species, leading to bioaccumulation of toxins by filter feeders, such as bivalves, with resultant negative impacts on physiological performance. In this study we manipulated multiple climate drivers (warming, freshening, and acidification), and the availability of toxic microalgae, to determine their impact on the physiological health, and toxin load of the tropical filter-feeding clam, Meretrix meretrix. Using a structural equation modeling (SEM) approach, we found that exposure to projected marine climates resulted in direct negative effects on metabolic and immunological function and, that these effects were often more pronounced in clams exposed to multiple, rather than single climate drivers. Furthermore, our study showed that these physiological responses were modified by indirect effects mediated through the food chain. Specifically, we found that when bivalves were fed with a toxin-producing dinoflagellate (Alexandrium minutum) the physiological responses, and toxin load changed differently and in a non-predictable way compared to clams exposed to projected marine climates only. Specifically, oxygen consumption data revealed that these clams did not respond physiologically to climate warming or the combined effects of warming, freshening and acidification. Our results highlight the importance of quantifying both direct and, indirect food chain effects of climate drivers on a key tropical food species, and have important implications for shellfish production and food safety in tropical regions.</p

Responses of shallow sediment ecosystems to environmental alterations

Shallow-water sediment systems are continuously exposed to a range of anthropogenic stressors, including increased nutrient loading, physical disturbance and toxicants. Superimposed on these local stressors are stressors related to the on-going climate change. Interacting stressors may strengthen or weaken effects of individual stressors, resulting in – often unexpected – non-additive effects. Thus, the combined effect from several simultaneous stressors in shallow-water sediment systems is the main topic of this thesis. Individual and combined stressor effects were studied on intact sediment cores placed in an outdoor flow-through facility and in the laboratory. Both functional (primary and bacterial production, community respiration, meiofaunal grazing, denitrification, nitrogen mineralization and sediment-water fluxes of oxygen and nutrients) and structural (biomass and composition of microphytobenthos and meiofauna) were studied in order to assess effects of multiple stressors. It has been suggested that global warming can shift the trophic status of ecosystems from net autotrophy to net heterotrophy. A spring experiment (Paper I) showed, however, that the presence and activity of a well-developed microphytobenthic mat already in early spring sustained net autotrophy during simulated warming. The effects of increased temperature on the structure and function of the sediment systems were generally rather moderate. Consequently, it is possible that the presence and activity of microphytobenthos can buffer changes induced by seawater temperature. An autumn experiment (Paper II) suggested that warming combined with nutrient enrichment can induce shorter, intensive, heterotrophic periods that can be followed by longer autotrophic periods because initially increased mineralization induces a shortage of labile organic matter. Even though warming increased nutrient availability through increased mineralization, warming did not exacerbate the stimulatory effects of nutrient enrichment. The lack of interactive temperature-nutrient effects was mainly explained by a sustained filter function by microphytobenthos. It has also been suggested that future warming will exacerbate the effect of toxicants. However, in paper III the combined effects of an antifouling compound (Cupper pyrithione) and warming resulted in antagonistic rather than synergistic effects. The two types of microalgal communities present at the end of the experiment, a typical benthic mat and a floating periphyhtic mat, were both significantly affected by the toxicant, but in opposite ways; the biomass of the benthic mat was stimulated by the toxicant, whereas the floating periphytic mat was negatively affected. Thus, autotrophic communities within the same ecosystem can respond differently. Also the nutrient status of an ecosystem can change toxicant effects. This was shown in paper IV, where the combination of pyrene (a polycyclic aromatic hydrocarbon) and nutrient enrichment synergistically reduced the grazing pressure of meiofauna on microphytobenthos, exerting a cascading effect on the primary producers. In paper V we studied whether herbivores can mediate effects of ocean warming and acidification on microphytobenthos in a seagrass meadow, and how these effects were related to the biomass of Zostera marina, filamentous macroalgae and sediment-associated fauna. Analysis of variance and structural equation modeling (SEM) were used to partition net effects of warming and ocean acidification into direct and indirect effects. This statistical approach showed that the absence of stressor effects on microphytobenthos was actually a combination of direct and indirect effects mediated via grazers on filamentous macroalgae and sediment associated fauna. Overall, the main results in my thesis is that even though changes in temperature, pH, toxin and nutrient availability, will occur in present time as well as the near future, the main function of shallow-water sediment ecosystems will be sustained given that microphytobenthos are present. My results also show that environmental alterations need to be studied together on an ecosystem level rather than on individual species level and that indirect effects always need to be taken into consideration when interpreting experimental results

stressordata and multifunctionality

<p>Individual stressordata on the effects of multifunctionality in shallow-water sediment ecosystems</p

Functioning of a shallow-water sediment system during experimental warming and nutrient enrichment.

Effects of warming and nutrient enrichment on intact unvegetated shallow-water sediment were investigated for 5 weeks in the autumn under simulated natural field conditions, with a main focus on trophic state and benthic nitrogen cycling. In a flow-through system, sediment was exposed to either seawater at ambient temperature or seawater heated 4°C above ambient, with either natural or nutrient enriched water. Sediment-water fluxes of oxygen and inorganic nutrients, nitrogen mineralization, and denitrification were measured. Warming resulted in an earlier shift to net heterotrophy due to increased community respiration; primary production was not affected by temperature but (slightly) by nutrient enrichment. The heterotrophic state was, however, not further strengthened by warming, but was rather weakened, probably because increased mineralization induced a shortage of labile organic matter. Climate-related warming of seawater during autumn could therefore, in contrast to previous predictions, induce shorter but more intensive heterotrophic periods in shallow-water sediments, followed by longer autotrophic periods. Increased nitrogen mineralization and subsequent effluxes of ammonium during warming suggested a preferential response of organisms driving nitrogen mineralization when compared to sinks of ammonium such as nitrification and algal assimilation. Warming and nutrient enrichment resulted in non-additive effects on nitrogen mineralization and denitrification (synergism), as well as on benthic fluxes of phosphate (antagonism). The mode of interaction appears to be related to the trophic level of the organisms that are the main drivers of the affected processes. Despite the weak response of benthic microalgae to both warming and nutrient enrichment, the assimilation of nitrogen by microalgae was similar in magnitude to rates of nitrogen mineralization. This implies a sustained filter function and retention capacity of nutrients by the sediment

Sedimentens roll för näringsomsättningen grunda vikar i norra Stigfjorden, Orust

Ett flertal regeringsuppdrag under åren 2007-2009 gavs till Länsstyrelsen i Västra Götalands län och Vattenmyndigheten i Västerhavets vattendistrikt för att minska näringsämnestillförseln och därmed motverka övergödning av Västerhavet: "Finn de områden som göder havet mest" (regeringsbeslut 26, 2006), "Inventera behovet av och möjligheterna till restaurering av övergödda havsvikar och kustnära sjöar i Västerhavets vattendistrikt" (regeringsuppdrag 51b, 2007), "Minskad påverkan på havsmiljön från enskilda avlopp" (regeringsuppdrag 51c, 2007). Inom regeringsuppdragen pekades det ut områden med högst näringsämnesbelastning, men även övergödningskänslighet. Stigfjorden/Kalvöfjorden mellan Orust och Tjörn pekades ut som ett näringsämnesbelastat område med hög känslighet. Flera åtgärdsinsatser görs nu för att minska näringsämnesbelastningen från avlopp och jordbruk till Stigfjorden (Länsstyrelsen Rapport 2010: 46 "Åtgärdsprogram för näringsbegränsning inom Stigfjordens och Kalvöfjodens avrinningsområden"). I detta projekt har vi gjort en baslinjestudie av omsättningen av näringsämnen i norra Stigfjorden för att bättre kunna bedöma effekterna av planerade åtgärder för minskad tillförsel av kväve och fosfor till fjorden. Rapporten redovisar en studie om de grunda sedimentbottnarnas funktion för näringsomsättningen i norra Stigfjorden och gjordes i mars-september 2011, med tre mer omfattande mätningar i maj/juni, juli och september. Projektet har genomförts av Göteborgs Universitet, Institutionen för Marin Ekologi och Kungliga Vetenskapsakademin på uppdrag av Länsstyrelsen i Västra Götalands lä

Sedimentens roll för näringsomsättningen grunda vikar i norra Stigfjorden, Orust

Ett flertal regeringsuppdrag under åren 2007-2009 gavs till Länsstyrelsen i Västra Götalands län och Vattenmyndigheten i Västerhavets vattendistrikt för att minska näringsämnestillförseln och därmed motverka övergödning av Västerhavet: "Finn de områden som göder havet mest" (regeringsbeslut 26, 2006), "Inventera behovet av och möjligheterna till restaurering av övergödda havsvikar och kustnära sjöar i Västerhavets vattendistrikt" (regeringsuppdrag 51b, 2007), "Minskad påverkan på havsmiljön från enskilda avlopp" (regeringsuppdrag 51c, 2007). Inom regeringsuppdragen pekades det ut områden med högst näringsämnesbelastning, men även övergödningskänslighet. Stigfjorden/Kalvöfjorden mellan Orust och Tjörn pekades ut som ett näringsämnesbelastat område med hög känslighet. Flera åtgärdsinsatser görs nu för att minska näringsämnesbelastningen från avlopp och jordbruk till Stigfjorden (Länsstyrelsen Rapport 2010: 46 "Åtgärdsprogram för näringsbegränsning inom Stigfjordens och Kalvöfjodens avrinningsområden"). I detta projekt har vi gjort en baslinjestudie av omsättningen av näringsämnen i norra Stigfjorden för att bättre kunna bedöma effekterna av planerade åtgärder för minskad tillförsel av kväve och fosfor till fjorden. Rapporten redovisar en studie om de grunda sedimentbottnarnas funktion för näringsomsättningen i norra Stigfjorden och gjordes i mars-september 2011, med tre mer omfattande mätningar i maj/juni, juli och september. Projektet har genomförts av Göteborgs Universitet, Institutionen för Marin Ekologi och Kungliga Vetenskapsakademin på uppdrag av Länsstyrelsen i Västra Götalands lä

Nitrogen mineralization measured as NH₄⁺ mobilization in closed sediment incubations (mineralization_inc) (A, B), net nitrogen mineralization calculated from fluxes of ammonium, nitrate+nitrite and denitrification (mineralization_flux) (C, D) and algal assimilation based on 80% of GPP (algal assim_80GPP) (E, F) at ambient nutrient levels (left panels) and with nutrients added (right panels), at ambient seawater temperature and in heated seawater.

<p>Shown are means ± SE (n = 4).</p

Daily sediment–water oxygen fluxes: Net primary production (NPP) (A, B), community respiration (CR) (C, D), gross primary production (GPP) (E, F) and 24-h net oxygen fluxes (G, H) at ambient nutrient level (left panels) and with nutrients added (right panels), at ambient seawater temperature and in heated seawater.

<p>Shown are means ± SE (n = 4).</p

Daily sediment–water fluxes for silica (A, B) and dissolved inorganic phosphorus (DIP) (C, D) at ambient nutrient level (left panels) and with nutrients added (right panels) at ambient seawater temperature and in heated seawater.

<p>Shown are means ± SE (n = 4).</p