RESPONSES OF THE PHYTOPLANKTON COMMUNITY GROWTH RATE TO NUTRIENT PULSES IN VARIABLE ESTUARINE ENVIRONMENTS

In estuaries, phytoplankton are exposed to rapidly changing conditions that may have profound effects on community structure and function. In these experiments, we evaluated the growth, productivity, and compositional responses of natural phytoplankton communities exposed to limiting nutrient additions and incubation conditions typical of estuarine habitats. Mesocosm bioassays were used to measure the short-term (2-day) growth rate, primary productivity, and group-specific biomass responses of the phytoplankton community in the Neuse River Estuary, North Carolina. A three-factor (mixing, sediment addition, and nutrient addition) experimental design was applied using 55-L mesocosm tanks. Growth rates were determined using the ¹⁴C photopigment radiolabeling method, and the abundance of algal groups was based on quantification of chemosystematic photopigments by HPLC. For Neuse River Estuary phytoplankton communities, stratified (nonmixed), turbid, and low-nitrate conditions favored increases in cryptomonad biomass. Mixed, turbid, high-nitrate conditions were favorable for increased primary productivity and chlorophytes, diatoms, and cyanobacteria. The highest community growth rates occurred under calm, high-nitrate conditions. This approach provided an assessment of the community-level phytoplankton responses and insights into the mechanisms driving blooms and bloom species in estuarine waters. The ability to rapidly alter growth rates to capitalize on conditions conducive for growth may play an important role in the timing, extent, and species involved with blooms in estuarine waters. Adaptive growth rate responses of individual species, as well as the community as a whole, further illustrate the sensitivity of estuarine ecosystems to excessive N inputs.     

Impact of faunal complexity on nutrient supply in field mesocosms from a spruce forest soil

A field study in an acidic spruce forest soil using soil mesocosms was conducted to investigate the effects of mesofauna and macrofauna on exchangeable cations, organic matter content, base saturation, and Ca-lactate extractable nutrients. In the field, intact soil monoliths were taken from the ground, defaunated by deep-freezing and wrapped in nets of various mesh-sizes to control immigration of different faunal size classes. The monoliths were then replanted in the field. Three types of treatments for the mesocosms were prepared: (1) microbiota only, (2) microbiota and mesofauna, (3) microbiota, mesofauna, and macrofauna (=complex fauna). After eight months the mesocosms and unmanipulated control plots (treatment 4) were destructively sampled and submitted to chemical analysis. Generally, the exchangeable base cations and Mn²⁺ showed higher contents with increasing faunal complexity, whereas the exchangeable acidic cations of Fe³⁺ and Al³⁺ decreased in the monoliths with complex fauna. These effects were significant for K⁺, Mg²⁺ and Mn²⁺ in the L/F-layer and for Ca²⁺, Mn²⁺, Al³⁺ and Fe³⁺ in the H-layer. As a possible explanation a rise of ion-binding sites in the course of enhanced humification processes is discussed.In the L/F-layer base cations showed higher concentrations in the monoliths with complex fauna as compared to the control plot, which contained intact roots. This might be due to nutrient uptake by roots in the control plot or enhanced mineralization in the monoliths with complex fauna, where roots had been cut.     

Feeding behaviour,predatory functional responses and trophic interactions of the invasive Chinese mitten crab (Eriocheir sinensis) and signal crayfish (Pacifastacus leniusculus)

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Making environmental DNA count

The arc of reception for a new technology or method – like the reception of new information itself – can pass through predictable stages, with audiences’ responses evolving from ‘I don't believe it’, through ‘well, maybe’ to ‘yes, everyone knows that’ to, finally, ‘old news’. The idea that one can sample a volume of water, sequence DNA out of it, and report what species are living nearby has experienced roughly this series of responses among biologists, beginning with the microbial biologists who developed genetic techniques to reveal the unseen microbiome. ‘Macrobial’ biologists and ecologists – those accustomed to dealing with species they can see and count – have been slower to adopt such molecular survey techniques, in part because of the uncertain relationship between the number of recovered DNA sequences and the abundance of whole organisms in the sampled environment. In this issue of Molecular Ecology Resources, Evans et al. ( 2015 ) quantify this relationship for a suite of nine vertebrate species consisting of eight fish and one amphibian. Having detected all of the species present with a molecular toolbox of six primer sets, they consistently find DNA abundances are associated with species’ biomasses. The strength and slope of this association vary for each species and each primer set – further evidence that there is no universal parameter linking recovered DNA to species abundance – but Evans and colleagues take a significant step towards being able to answer the next question audiences tend to ask: ‘Yes, but how many are there?’     

Field assessment of the potential of algicidal bacteria against diatom blooms

The possible use of algicidal bacteria for the efficient termination of natural freshwater diatom blooms with minimal adverse effects on the freshwater ecosystem was assessed under laboratory and field conditions. A field mesocosm (150 L) was dosed with a single application of isolate SK09, and monitored at Samnang jin in the lower part of the Nakdong River (South Korea) over 12 days of the winter season. We found that the tested bacterium acted against some species of Stephanodiscus- and Aulacoseira-like structures on in vitro and in vivo diatom blooms. However, this bacterium failed to fully control in vivo natural blooms of Stephanodiscus due to the low water temperatures of less than 10°C and predation activity of protozooplankton (heterotrophic nanoflagellates and ciliates). In addition, its selective inhibition indirectly affected the decrease of dissolved oxygen levels, the dramatic regeneration of N and P by the large-scale Stephanodiscus-lysing process, and a great increase in algal biomass of genera Chlamydomonas. This strongly suggests the necessity of developing an effective strategy for enhancing the activity of algicidal bacteria, and for mitigating some drawbacks to effectively and safely regulate natural diatom blooms.     

Temporal stability vs. community matrix measures of stability and the role of weak interactions

Relationships between different measures of stability are not well understood in part because empiricists and theoreticians tend to measure different aspects and most studies only explore a single form of stability. Using time‐series data from experimental plankton communities, we compared temporal stability typically measured by empiricists (coefficient of variation in biomass) to stability measures typically measured by theoreticians derived from the community matrix (asymptotic resilience, initial resilience and intrinsic stochastic invariability) using first‐order multivariate autoregressive models (MAR). Community matrices were also used to derive estimates of interaction strengths between plankton groups. We found no relationship between temporal stability and stability measures derived from the community matrix. Weaker interaction strengths were generally associated with higher stability for community matrix measures of stability, but were not consistently associated with higher temporal stability. Temporal stability and stability measures derived from the community matrix stability appear to represent different aspects of stability reflecting the multi‐dimensionality of stability.     

Nutrient Addition to Experimental Rocky Shore Communities Revisited: Delayed Responses, Rapid Recovery

Coastal eutrophication may alter the dominance patterns of marine macroalgae, with potential consequences for the associated fauna and the entire ecosystem. Benthic macroalgae and animals in control and nutrient-enriched mesocosms were monitored to investigate eutrophication-induced changes in rocky shore communities. During a 3-year project, nutrient addition had only minor effects on the community structure, such as increased cover and biomass of green Ulva spp. and increased abundance of certain animal species at high nutrient levels. This study is a 4-year extension of a previously reported project, with 2 extra years of effect studies (altogether 5 years) and a subsequent 2 years for recovery. During the 4th year of nutrient enrichment, the cover of Fucus vesiculosus and Fucus serratus started to decline. In the 5th year, these canopy species crashed and there was an evident take-over by green algae at high nutrient addition levels. The previously observed abundance stimulation for fauna disappeared later in the time series, probably due to the loss of the macroalgal canopy. After less than 2 years on regular seawater, the algal and animal communities had returned to within the range of normal variability. The results indicate that established rocky shore communities of perennial algae with associated fauna are able to persist for several years, even at very high nutrient levels, but that community shifts may suddenly occur if eutrophication continues. They also indicate that rocky shore communities have the ability to return rapidly to natural undisturbed conditions after the termination of nutrient enhancement.     

Responses of phytoplankton to fish predation and nutrient loading in shallow lakes: a pan-European mesocosm experiment

1. The impacts of nutrients (phosphorus and nitrogen) and planktivorous fish on phytoplankton composition and biomass were studied in six shallow, macrophyte‐dominated lakes across Europe using mesocosm experiments. 2. Phytoplankton biomass was more influenced by nutrients than by densities of planktivorous fish. Nutrient addition resulted in increased algal biomass at all locations. In some experiments, a decrease was noted at the highest nutrient loadings, corresponding to added concentrations of 1 mg L^?1 P and 10 mg L^?1 N. 3. Chlorophyll a was a more precise parameter to quantify phytoplankton biomass than algal biovolume, with lower within‐treatment variability. 4. Higher densities of planktivorous fish shifted phytoplankton composition toward smaller algae (GALD < 50 μm). High nutrient loadings selected in favour of chlorophytes and cyanobacteria, while biovolumes of diatoms and dinophytes decreased. High temperatures also may increase the contribution of cyanobacteria to total phytoplankton biovolume in shallow lakes.     

Differential survival of two minnow species under experimental sunfish predation: implications for re‐invasion of a species into its native range

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Ecosystem carbon response of an Arctic peatland to simulated permafrost thaw

Permafrost peatlands are biogeochemical hot spots in the Arctic as they store vast amounts of carbon. Permafrost thaw could release part of these long‐term immobile carbon stocks as the greenhouse gases (GHGs) carbon dioxide (CO₂) and methane (CH₄) to the atmosphere, but how much, at which time‐span and as which gaseous carbon species is still highly uncertain. Here we assess the effect of permafrost thaw on GHG dynamics under different moisture and vegetation scenarios in a permafrost peatland. A novel experimental approach using intact plant–soil systems (mesocosms) allowed us to simulate permafrost thaw under near‐natural conditions. We monitored GHG flux dynamics via high‐resolution flow‐through gas measurements, combined with detailed monitoring of soil GHG concentration dynamics, yielding insights into GHG production and consumption potential of individual soil layers. Thawing the upper 10–15 cm of permafrost under dry conditions increased CO₂ emissions to the atmosphere (without vegetation: 0.74 ± 0.49 vs. 0.84 ± 0.60 g CO₂–C m^?2 day^?1; with vegetation: 1.20 ± 0.50 vs. 1.32 ± 0.60 g CO₂–C m^?2 day^?1, mean ± SD, pre‐ and post‐thaw, respectively). Radiocarbon dating (¹⁴C) of respired CO₂, supported by an independent curve‐fitting approach, showed a clear contribution (9%–27%) of old carbon to this enhanced post‐thaw CO₂ flux. Elevated concentrations of CO₂, CH₄, and dissolved organic carbon at depth indicated not just pulse emissions during the thawing process, but sustained decomposition and GHG production from thawed permafrost. Oxidation of CH₄ in the peat column, however, prevented CH₄ release to the atmosphere. Importantly, we show here that, under dry conditions, peatlands strengthen the permafrost–carbon feedback by adding to the atmospheric CO₂ burden post‐thaw. However, as long as the water table remains low, our results reveal a strong CH₄ sink capacity in these types of Arctic ecosystems pre‐ and post‐thaw, with the potential to compensate part of the permafrost CO₂ losses over longer timescales.