Posts Tagged 'community composition'

Abiotic drivers of interannual phytoplankton variability and a 1999–2000 regime shift in the North Sea examined by multivariate statistics

The Dutch coastal zone is a region of the North Sea with a marked interannual and long‐term abiotic and phytoplankton variability. To investigate the relationship between abiotic variability and phytoplankton composition, two routine water monitoring data sets (1991–2005) were examined. Multivariate statistics revealed two significant partitions in the data. The first consisted of interannual abiotic fluctuations that were correlated to Rhine discharge that affected the abundance of summer and autumn diatom species. The second partition was caused by a shift in the abiotic data from 1998 to 1999 that was followed by a shift in phytoplankton composition from 1999 to 2000. Important factors in the abiotic shift were decreases in suspended matter (SPM) and phosphate (DIP) concentrations, as well as in pH. The decrease in SPM was caused by a reduction in wind speed. The increase in water column daily irradiance from the decrease in SPM led to increases in the abundance of winter–spring species, notably the prymnesiophyte Phaeocystis globosa. Because wind speed is related to the North Atlantic Oscillation (NAO) index it was possible to correlate NAO index and P. globosa abundance. Only five abiotic variables representing interannual and long‐term variability, including Rhine discharge and NAO index, were needed to model the observed partitions in phytoplankton composition. It was concluded that interannual variability in the coastal phytoplankton composition was related to year‐to‐year changes in river discharge while the long‐term shift was caused by an alternating large‐scale meteorological phenomenon.

Continue reading ‘Abiotic drivers of interannual phytoplankton variability and a 1999–2000 regime shift in the North Sea examined by multivariate statistics’

Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions

Global degradation of coral reefs has increased the urgency of identifying stress-tolerant coral populations, to enhance understanding of the biology driving stress tolerance, as well as identifying stocks of stress-hardened populations to aid reef rehabilitation. Surprisingly, scientists are continually discovering that naturally extreme environments house established coral populations adapted to grow within extreme abiotic conditions comparable to seawater conditions predicted over the coming century. Such environments include inshore mangrove lagoons that carry previously unrecognised ecosystem service value for corals, spanning from refuge to stress preconditioning. However, the existence of such hot-spots of resilience on the Great Barrier Reef (GBR) remains entirely unknown. Here we describe, for the first time, 2 extreme GBR mangrove lagoons (Woody Isles and Howick Island), exposing taxonomically diverse coral communities (34 species, 7 growth morphologies) to regular extreme low pH (<7.6), low oxygen (7°C) conditions. Coral cover was typically low (0.5 m diameter), with net photosynthesis and calcification rates of 2 dominant coral species (Acropora millepora, Porites lutea) reduced (20-30%), and respiration enhanced (11-35%), in the mangrove lagoon relative to adjacent reefs. Further analysis revealed that physiological plasticity (photosynthetic ‘strategy’) and flexibility of Symbiodiniaceae taxa associations appear crucial in supporting coral capacity to thrive from reef to lagoon. Prevalence of corals within these extreme conditions on the GBR (and elsewhere) increasingly challenge our understanding of coral resilience to stressors, and highlight the need to study unfavourable coral environments to better resolve mechanisms of stress tolerance.

Continue reading ‘Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions’

CO2 effects on diatoms: a synthesis of more than a decade of ocean acidification experiments with natural communities (update)

Diatoms account for up to 50 % of marine primary production and are considered to be key players in the biological carbon pump. Ocean acidification (OA) is expected to affect diatoms primarily by changing the availability of CO2 as a substrate for photosynthesis or through altered ecological interactions within the marine food web. Yet, there is little consensus how entire diatom communities will respond to increasing CO2. To address this question, we synthesized the literature from over a decade of OA-experiments with natural diatom communities to uncover the following: (1) if and how bulk diatom communities respond to elevated CO2 with respect to abundance or biomass and (2) if shifts within the diatom communities could be expected and how they are expressed with respect to taxonomic affiliation and size structure. We found that bulk diatom communities responded to high CO2 in ∼60 % of the experiments and in this case more often positively (56 %) than negatively (32 %) (12 % did not report the direction of change). Shifts among different diatom species were observed in 65 % of the experiments. Our synthesis supports the hypothesis that high CO2 particularly favours larger species as 12 out of 13 experiments which investigated cell size found a shift towards larger species. Unravelling winners and losers with respect to taxonomic affiliation was difficult due to a limited database. The OA-induced changes in diatom competitiveness and assemblage structure may alter key ecosystem services due to the pivotal role diatoms play in trophic transfer and biogeochemical cycles.

Continue reading ‘CO2 effects on diatoms: a synthesis of more than a decade of ocean acidification experiments with natural communities (update)’

Calcium carbonate alters the functional response of coastal sediments to eutrophication-induced acidification

Coastal ocean acidification research is dominated by laboratory-based studies that cannot necessarily predict real-world ecosystem response given its complexity. We enriched coastal sediments with increasing quantities of organic matter in the field to identify the effects of eutrophication-induced acidification on benthic structure and function, and assess whether biogenic calcium carbonate (CaCO3) would alter the response. Along the eutrophication gradient we observed declines in macrofauna biodiversity and impaired benthic net primary productivity and sediment nutrient cycling. CaCO3 addition did not alter the macrofauna community response, but significantly dampened negative effects on function (e.g. net autotrophy occurred at higher levels of organic matter enrichment in +CaCO3 treatments than −CaCO3 (1400 vs 950 g dw m−2)). By identifying the links between eutrophication, sediment biogeochemistry and benthic ecosystem structure and function in situ, our study represents a crucial step forward in understanding the ecological effects of coastal acidification and the role of biogenic CaCO3 in moderating responses.

Continue reading ‘Calcium carbonate alters the functional response of coastal sediments to eutrophication-induced acidification’

Effects of reduced seawater pH on nematode community composition and diversity in sandy sediments

Highlights

• Reduced pH changed nematode community composition in medium sand sediment.

• Reduced pH increased nematode diversity in medium sand sediments.

• Proportion of nematodes with higher tolerance to lowered pH increased in medium sand.

• Nematode communities in finer sediments appeared less sensitive to reduced pH.

Abstract

The present study investigated the potential effects of seawater acidification on the taxonomic structure and diversity of nematode communities using a microcosm experiment. Nematode samples for the microcosm experiment were collected from the low tidal zone of two sandy beaches with different sediment compositions (medium sand vs. very fine sand) in Qingdao (China). In the microcosm, nematode communities were exposed to nine experimental treatments comprising two pH levels for 56 days: 8.0 (ambient control) and 7.3. Communities were exposed for 0, 7, 14, 28, or 56 days. Results showed that the most distinguishable differences in nematode community structure and diversity indices were caused by sediment type. Reduced pH changed the taxonomic structure of nematode communities in medium sand sediments. An increase in species with higher tolerance to lowered pH occurred as a response and resulted in increased diversity in medium sand sediments. Nematode communities in finer sediments appeared less sensitive to reduced pH.

Continue reading ‘Effects of reduced seawater pH on nematode community composition and diversity in sandy sediments’

Episodic Arctic CO2 limitation in the west Svalbard shelf

The European Sector of the Arctic Ocean is characterized by low CO2 concentrations in seawater during spring and summer, largely due to strong biological uptake driven by extensive plankton blooms in spring. The spring plankton bloom is eventually terminated by nutrient depletion and grazing. However, low CO2 concentrations in seawater and low atmospheric resupply of CO2 can cause episodes during which the phytoplankton growth is limited by CO2. Here, we show that gross primary production (GPP) of Arctic plankton communities increases from 32 to 72% on average with CO2 additions in spring. Enhanced GPP with CO2 additions occur during episodes of high productivity, low CO2 concentration and in the presence of dissolved inorganic nutrients. However, during summer the addition of CO2 supresses planktonic Arctic GPP. Events of CO2 limitation in spring may contribute to the termination of the Arctic spring plankton blooms. The stimulation of GPP by CO2 during the spring bloom provides a biotic feedback loop that might influence the global role played by the Arctic Ocean as a CO2 sink in the future.

Continue reading ‘Episodic Arctic CO2 limitation in the west Svalbard shelf’

Effects of ocean acidification and short-term light/temperature stress on biogenic dimethylated sulfur compounds cycling in the Changjiang River Estuary

Ocean acidification (OA) affects marine primary productivity and community structure. Therefore, OA may influence the biogeochemical cycles of volatile biogenic dimethyl sulfide (DMS), and its precursor dimethylsulfoniopropionate (DMSP) and photochemical oxidation product dimethyl sulfoxide (DMSO). A 23-day shipboard incubation experiment investigated the short-term response of the production and cycling of biogenic sulfur compounds to OA in the Changjiang River Estuary to understand the effects of OA on biogenic sulfur compounds. Phytoplankton abundance and community composition showed a marked difference at three different pH levels at the late stage of the experiment. Significant reductions in chlorophyll a (Chl-a), DMS, particulate DMSP (DMSPp) and dissolved DMSO (DMSOd) concentrations were identified under high CO2 levels. Moreover, minimal changes were observed in the productions of dissolved DMSP (DMSPd) and particulate DMSO (DMSOp) among the treatments. The ratios of DMS, total DMSP (DMSPt) and total DMSO (DMSOt) to Chl-a were not affected by a change in pH. Furthermore, the concentrations of DMS and DMSOd were closely related to the mean bacterial abundance at the three pH levels. Additional short-term (8 h) incubation experiments on the light and temperature effects showed that the influence of pH on the production of dimethylated sulfur compounds also depended on solar radiation and temperature. Under natural and UVB light, DMS photodegradation rates increased by 1.6 to 4.2 times at low pH levels. Thus, OA may lead to decreasing DMS concentrations in surface seawater. Light and temperature conditions also play important roles in the production and cycling of biogenic sulfur compounds.

Continue reading ‘Effects of ocean acidification and short-term light/temperature stress on biogenic dimethylated sulfur compounds cycling in the Changjiang River Estuary’


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Ocean acidification in the IPCC AR5 WG II

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