A biological consequence of reducing Arctic ice cover: arrival of the Pacific diatom Neodenticula seminae in the North Atlantic for the first time in 800 000 years

The Continuous Plankton Recorder survey has monitored plankton in the Northwest Atlantic at monthly intervals since 1962, with an interegnum between 1978 and 1990. In May 1999, large numbers of the Pacific diatom Neodenticula seminae were found in Continuous Plankton Recorder (CPR) samples in the Labrador Sea as the first record in the North Atlantic for more than 800 000 years. The event coincided with modifications in Arctic hydrography and circulation, increased flows of Pacific water into the Northwest Atlantic and in the previous year the exceptional occurrence of extensive ice‐free water to the North of Canada. These observations indicate that N. seminae was carried in a pulse of Pacific water in 1998/early 1999 via the Canadian Arctic Archipelago and/or Fram Strait. The species occurred previously in the North Atlantic during the Pleistocene from∼1.2 to∼0.8 Ma as recorded in deep sea sediment cores. The reappearance of N. seminae in the North Atlantic is an indicator of the scale and speed of changes that are taking place in the Arctic and North Atlantic oceans as a consequence of regional climate warming. Because of the unusual nature of the event it appears that a threshold has been passed, marking a change in the circulation between the North Pacific and North Atlantic Oceans via the Arctic. Trans‐Arctic migrations from the Pacific into the Atlantic are likely to occur increasingly over the next 100 years as Arctic ice continues to melt affecting Atlantic biodiversity and the biological pump with consequent feedbacks to the carbon cycle.     

A study of genetic diversity in Populus nigra subsp. betulifolia in the Upper Severn area of the UK using AFLP markers

A survey of amplified fragment length polymorphism (AFLP) was conducted to examine genetic diversity in 146 individuals of British, native black poplar ( Populus nigra subsp. betulifolia ) and three individuals considered by collectors to be non- betulifolia poplars. Using two primer pairs, a total of 147 bands were detected of which 82 (56%) were polymorphic in at least one individual. Cluster analysis and principal coordinates analysis of the calculated similarity matrix revealed a low level of genetic diversity, although a loose clustering into five groups could be identified, one of which contained the non- betulifolia individuals. Examination of the spatial distribution of the other four groups (all betulifolia ) revealed a general correlation between geographic proximity and genetic similarity. On the basis of the polymorphism observed, it was possible to identify a small number of individual plants which exhibit maximum diversity and might therefore be suitable for inclusion in a replanting programme designed to maintain at least the existing low level of polymorphism observed within British black poplar.     

A new group of Early Devonian plants with valvate sporangia containing sculptured permanent dyads

As part of a study to explore diversity and disparity in Early Devonian terrestrial vegetation, several hundreds of sporangia with in situ spores have been isolated from a Lochkovian locality in Shropshire. These include a small number (seven) of sporangia showing dehiscence into four valves and containing permanent sculptured dyads, belonging to the Cymbohilates horridus complex and C. cymosus, which are recorded in coeval dispersed spore assemblages. A further, previously described, mesofossil comprises an incomplete sporangium containing C. horridus that terminates a naked isotomously branching stem with stomata. The valvate sporangia are placed in a new genus, Partitatheca, containing four species, P. splendida (type), P. horrida, P. densa and P. cymosa, their names reflecting the names of the dispersed spore species and varieties. Complex ultrastructure in the walls of the dyads is similar to that in earlier dyads in the Dyadospora complex where it provides evidence for a hepatic affinity of the earliest embryophytes, but the new taxa present a combination of bryophyte and tracheophyte characters and are considered to represent a new embryophyte lineage. General discussion includes the development of dyads, more particularly their relevance to understanding the diversity in meiotic processes, and the disappearance of dyads from the dispersed spore record prior to the Middle Devonian. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 168 , 229–257.     

Exoenzyme activities as indicators of dissolved organic matter composition in the hyporheic zone of a floodplain river

1. We measured the hyporheic microbial exoenzyme activities in a floodplain river to determine whether dissolved organic matter (DOM) bioavailability varied with overlying riparian vegetation patch structure or position along flowpaths. 2. Particulate organic matter (POM), dissolved organic carbon (DOC), dissolved oxygen (DO), electrical conductivity and temperature were sampled from wells in a riparian terrace on the Queets River, Washington, U.S.A. on 25 March, 15 May, 20 July and 09 October 1999. Dissolved nitrate, ammonium and soluble reactive phosphorus were also collected on 20 July and 09 October 1999. Wells were characterised by their associated overlying vegetation: bare cobble/young alder, mid‐aged alder (8–20 years) and old alder/old‐growth conifer (25 to >100 years). POM was analysed for the ash‐free dry mass and the activities of eight exoenzymes (α‐glucosidase, β‐glucosidase, β ‐N‐acetylglucosaminidase, xylosidase, phosphatase, leucine aminopeptidase, esterase and endopeptidase) using fluorogenic substrates. 3. Exoenzyme activities in the Queets River hyporheic zone indicated the presence of an active microbial community metabolising a diverse array of organic molecules. Individual exoenzyme activity (mean ± standard error) ranged from 0.507 ± 0.1547 to 22.8 ± 5.69 μmol MUF (g AFDM)^?1 h^?1, was highly variable among wells and varied seasonally, with the lowest rates occurring in March. Exoenzyme activities were weakly correlated with DO, DOC and inorganic nutrient concentrations. 4. Ratios of leucine aminopeptidase : β‐glucosidase were low in March, May and October and high in July, potentially indicating a switch from polysaccharides to proteins as the dominant component of microbial metabolism. 5. Principal components analysis indicated that there were patch effects and that these effects were strongest in the summer. 6. DOM degradation patterns did not change systematically along hyporheic flowpaths but varied with overlying forest patch type in the Queets River hyporheic zone, suggesting that additional carbon inputs exist. We hypothesise that the most likely input is the downward movement of DOM from overlying riparian soils. Understanding this movement of DOM from soils to subsurface water is essential for understanding both the hyporheic metabolism and the carbon budget of streams and rivers.     

Temperature-independent diel variation in soil respiration observed from a temperate deciduous forest

The response of soil respiration (R_s) to temperature depends largely on the temporal and spatial scales of interest and how other environmental factors interact with this response. They are often represented by empirical exponential equations in many ecosystem analyses because of the difficulties in separating covarying environmental responses and in observing below ground processes. The objective of this study was to quantify a soil temperature‐independent component in R_s by examining the diel variation of an R_s time series measured in a temperate deciduous forest located at Oak Ridge, TN, USA between March and December 2003. By fitting 2 hourly, continuous automatic chamber measurements of CO₂ efflux at the soil surface to a Q₁₀ function to obtain the temperature‐dependent respiration (R_t) and plotting the diel cycles of R_t, R_s, and their difference (R_i), we found that an obvious temperature‐independent component exists in R_s during the growing season. The diel cycle of this component has a distinct day/night pattern and agrees well with diel variations in photosynthetically active radiation (PAR) and air temperature. Elevated canopy CO₂ concentration resulted in similar patterns in the diel cycle of the temperature‐independent component but with different daily average rates in different stages of growing season. We speculate that photosynthesis of the stand is one of the main contributors to this temperature‐independent respiration component although more experiments are needed to draw a firm conclusion. We also found that despite its relatively small magnitude compared with the temperature‐dependent component, the diel variation in the temperature‐independent component can lead to significantly different estimates of the temperature sensitivity of soil respiration in the study forest. As a result, the common practice of using fitted temperature‐dependent function from night‐time measurements to extrapolate soil respiration during the daytime may underestimate daytime soil respiration.