Tropical herbivores provide resilience to a climate‐mediated phase shift on temperate reefs

Climate‐mediated changes to biotic interactions have the potential to fundamentally alter global ecosystems. However, the capacity for novel interactions to drive or maintain transitions in ecosystem states remains unresolved. We examined temperate reefs that recently underwent complete seaweed canopy loss and tested whether a concurrent increase in tropical herbivores could be maintaining the current canopy‐free state. Turf‐grazing herbivorous fishes increased in biomass and diversity, and displayed feeding rates comparable to global coral reefs. Canopy‐browsing herbivores displayed high (~ 10 000 g 100 m^?2) and stable biomass between 2006 and 2013. Tropical browsers had the highest abundance in 2013 and displayed feeding rates approximately three times higher than previously observed on coral reefs. These observations suggest that tropical herbivores are maintaining previously kelp‐dominated temperate reefs in an alternate canopy‐free state by grazing turfs and preventing kelp reestablishment. This remarkable ecosystem highlights the sensitivity of biotic interactions and ecosystem stability to warming and extreme disturbance events.     

Effect of feeding and sludge age on acclimated bacterial community and fate of slowly biodegradable substrate

The study investigated the effect of feeding regime and sludge age on starch utilization. For this purpose, parallel sequencing batch reactors were operated with pulse and continuous feeding of soluble starch at sludge ages of 8 and 2 days. Pulse feeding induced almost complete conversion of starch to glycogen, while storage was lowered and accompanied with direct growth under continuous feeding, regardless of sludge age. Low sludge age did not alter simultaneous storage and utilization for direct growth but it slightly favoured direct utilization due to faster growing biomass. Experimental results suggested adsorption of starch onto biomass as a preliminary removal mechanism prior to hydrolysis at sludge age of 8 days. Adsorption was not noticeable as substrate removal, glycogen generation and dissolved oxygen decrease were synchronous at sludge age of 2 days. Bacterial community always included fractions storing glycogen although sludge age only affected the relative magnitude of filamentous growth.     

Estimating biogeochemical and biotic interactions between a stream channel and a created riparian wetland: A medium-scale physical model

Straightened channels and altered and drained adjacent riparian wetlands have adversely impacted streams and rivers throughout the US Midwest. This research investigated the biological connection and water quality of a 0.07 ha diversion wetland and adjacent stream at the Olentangy River Wetland Research Park in central Ohio. Before the flowthrough conditions were established, we demonstrated with mark and recapture techniques that the wetland already was a biorefuge for fish under extreme conditions; two species (Centrarchidae) captured in the stream before a total drawdown of the stream were found in the wetland a year later. In addition, water at the bottom remained at around 4 °C over the winter likely due to groundwater input, which possibly provided a warmer shelter for fish. Stream water quality of the lower section, downstream of the wetland outlet, generally improved with hydrologic pulsing in spring after flow-through reconnection due to the trapping of nutrients in the wetland. Mean removal per flood pulse for nitrate-nitrite, total nitrogen (TN), soluble reactive phosphorus (SRP), total phosphorus (TP) were 1.81 g-N m⁻² per pulse, 1.02 g-N m⁻² per pulse, 0.014 g-P m⁻² per pulse, and 0.004 g-P m⁻² per pulse, respectively. The wetland exported 2.8 g-C m⁻² per pulse of organic carbon. A greater attenuation of NO₃⁻ and TP occurred in the marshy outlet channel section of the wetland than the open water section. The diversion wetland successfully removed nitrate and phosphorus during storm pulses in spring. Similar designs should be applied to other locations to examine their function under different climatic and hydrological conditions.     

Climate change impact on Balearic shearwater through a trophic cascade

A recent study showed that a critically endangered migratory predator species, the Balearic shearwater Puffinus mauretanicus, rapidly expanded northwards in northeast Atlantic waters after the mid-1990s. As a significant positive correlation was found between the long-term changes in the abundance of this seabird and sea temperature around the British Isles, it was hypothesized that the link between the biogeographic shift and temperature occurred through the food web. Here, we test this conjecture and reveal concomitant changes in a regional index of sea temperature, plankton (total calanoid copepod), fish prey (anchovy and sardine) and the Balearic shearwater for the period 1980-2003. All three trophic levels exhibit a significant shift detected between 1994 and 1996. Our findings therefore support the assertion of both a direct and an indirect effect of climate change on the spatial distribution of post-breeding Balearic shearwater through a trophic cascade.     

Hydrodynamics and mass transfer coefficient in activated sludge aerated stirred column reactor: experimental analysis and modeling

The aerated stirred reactor (ASR) has been widely used in biochemical and wastewater treatment processes. The information describing how the activated sludge properties and operation conditions affect the hydrodynamics and mass transfer coefficient is missing in the literature. The aim of this study was to investigate the influence of flow regime, superficial gas velocity (U(G)), power consumption unit (P/V(L)), sludge loading, and apparent viscosity (mu(ap)) of activated sludge fluid on the mixing time (t(m)), gas hold-up (epsilon), and volumetric mass transfer coefficient (k(L)a) in an activated sludge aerated stirred column reactor (ASCR). The activated sludge fluid performed a non-Newtonian rheological behavior. The sludge loading significantly affected the fluid hydrodynamics and mass transfer. With an increase in the U(G) and P/V(L), the epsilon and k(L)a increased, and the t(m), decreased. The epsilon, k(L)a, and t(m), were influenced dramatically as the flow regime changed from homogeneous to heterogeneous patterns. The proposed mathematical models predicted the experimental results well under experimental conditions, indicating that the U(G), P/V(L), and mu(ap) had significant impact on the t(m), epsilon, and k(L)a. These models were able to give the t(m), epsilon, and k(L)a values with an error around +/-8%, and always less than +/-10%.     

Translocation and Spread of Piscivorous Fishes in the Burdekin River, North-eastern Australia

The distribution of the biogeographically distinctive fish fauna of the Burdekin River, north-eastern Australia, is largely determined by the presence of a large waterfall located at the lower quarter of the river’s length. Downstream of the falls, assemblages are characterised by the presence of piscivorous fishes whereas such species are largely absent from upstream reaches. Sleepy cod (Oxyeleotris lineolatus), a large piscivorous gudgeon, was first introduced into the upper reaches of the Burdekin River in 1980 and other releases, both official and unofficial, have occurred subsequently. The population remained small and restricted to the site of introduction for a decade, but expanded in size and distribution after the occurrence of a large flood and entry into a prolonged period of drought. This gudgeon is now present in every tributary system of the Burdekin Basin. Despite the occurrence of substantial temporal variation in fish abundance due to a highly variable flow regime, negative impacts on one species, a small gudgeon (Mogurnda adspersa), are evident. Both deliberate and accidental releases of other species into the upper Burdekin River have also occurred, often to satisfy recreational fishing demand. Such species are typified by large size and piscivorous habit, characteristics alien and inimical to the native fish fauna. It is hypothesised that these piscivorous species may have even greater impact than O. lineolatus in some tributary systems of the upper Burdekin River.     

Foraging guild membership explains variation in waterbird responses to the hydrological regime of an arid‐region flood‐pulse river in Namibia

1. Little is known about hydrological influences on tropical waterbird communities. We used a 16‐year data set (1991–2007) of waterbird censuses, together with a classification of observed species into foraging guilds, to explore the relationships between natural variations in flow regime, foraging guild and the community composition of waterbirds at the Okavango River in the Caprivi Strip of north‐eastern Namibia, southern Africa. 2. We addressed three hypotheses to explain variation in waterbird community composition: (i) exploitation (birds move towards resource‐rich patches to exploit periods of high food abundance); (ii) escapism (declines in regional habitat quality force birds to aggregate in perennial waterbodies); and (iii) interaction (bird assemblages are dominated by intra‐ and interspecific interactions, such as flock formation for breeding or moulting, that can be explained better by life history demands or competition than by resource availability). 3. Waterbirds in different foraging guilds responded strongly but at different periods to changes in the hydrological environment, creating a complex but predictable successional pattern in community composition through time. Deep‐water feeders responded fastest (abundance peaking 2 months post‐flood), followed by shallow‐water feeders (4 months) and emergent vegetation feeders (7 months). Species that forage on short vegetation or in mud showed a bimodal response with peaks in abundance at 3 and 8 months post‐flood. 4. Our results indicated a strong effect of the local flow regime and hence supported the exploitation hypothesis. The foraging guild approach allowed us to identify clear patterns in a highly complex ecosystem and shows considerable promise as an analytical tool for similar data sets. Our results further suggest that while the entire bird community will be affected by hydrological alterations such as impoundments, water extraction and climate change, deep‐water feeders may be one of the most vulnerable groups.