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Widespread changes in the relative abundances of Cyclotella sensu lato taxa have occurred in lakes of the Northern Hemisphere over the last ∼150 years, with these changes often attributed to climate-driven physical changes in lakes. While the links between Cyclotella ecology and lake thermal structure have been investigated extensively now, the role of the timing of ice-off in driving these diatom patterns remains unclear. We compared the seasonal distribution patterns of Cyclotella taxa in a boreal lake with high water transparency during two years with differing timing of ice-off. Vertical distributions of taxa in Jordan Pond (Maine, USA) were assessed every 5–24 days from early May to late October during an early ice-off year (19 March 2012) and a late ice-off year (29 April 2015). Seasonal succession patterns differed over the two years. During the early ice-off year, Lindavia bodanica cell densities were higher and peaked in the hypolimnion from July to October. The abundance of this taxon during the...

In boreal regions, increased concentrations of dissolved organic carbon (DOC) have been linked to extreme wet years; however, less is known about the extent to which precipitation events are altering DOC concentration and quality. We assessed the effects of rain events on a suite of six lakes in Maine, U.S.A., to better understand how events alter DOC quantity and quality. DOC concentrations and DOC quality (measured as DOC-specific absorption coefficients (Specific Ultraviolet Absorbance (SUVA254 (also a*254), a*320, and a*380)) were quantified 24 h before, and at three time points (24–48 h, 5–7 days, and 3 weeks) after five different precipitation events. Our results revealed three types of responses across the lakes: (1) an initial spike in DOC concentrations of 30–133% and in the three quality metrics of 20–86% compared to pre-storm levels, followed by return to pre-storm concentrations; (2) a sustained increase in DOC concentrations (by 4–23%) and an increase in the three DOC quality metrics (by 1–43%) through the second post-storm sampling, with concentrations falling by the third post-storm sampling compared to pre-storm levels; and (3) no change during all sampling periods. Lake residence time was a key driver of changes in DOC concentration and DOC quality in response to storm events. Our research provides evidence that precipitation events contribute to short-term abrupt changes in DOC quantity and quality that are largely driven by key landscape and lake characteristics. These changes in DOC may have important implications for management of water utilities, including alteration or implementation of treatment strategies.

In boreal lakes, increased precipitation events have been linked to increased concentrations of dissolved organic carbon (DOC), however the effects of seasonal differences on DOC and how this may impact storm response remain unclear. We evaluated DOC concentration and a set of DOC quality metrics during an early summer storm and an autumn storm on a suite of six lakes in Acadia National Park in Maine, USA. to better understand differences in seasonal storm responses. Our results revealed differences in the response of DOC quality metrics to an early summer versus an autumn storm, with changes in DOC quality metrics varying by storm and lake features. During the early summer storm, we observed greater changes in various DOC quality metrics in deep lakes with longer residence times, whereas during the autumn storm, lakes with large watershed area to lake area ratios experienced the greatest changes. Land cover was highly correlated with changing DOC quality metrics in the early summer storm but did not play a significant role in the autumn storm response. Our research provides evidence of seasonal differences in the effects of storms on boreal lakes, which are ultimately mediated by a combination of lake and watershed characteristics as well as seasonal differences in climate such as solar radiation and antecedent weather conditions.

The timing of lake ice-out has advanced substantially in many regions of the Northern Hemisphere, however the effects of ice-out timing on lake properties and how they vary regionally remain unclear. Using data from two inter-annual monitoring datasets for a set of three Arctic lakes and one boreal lake, we compared physical, chemical and phytoplankton metrics from two years in which ice-out timing differed by at least three weeks. Our results revealed regional differences in lake responses during early compared to late ice-out years. With earlier ice-out, Arctic lakes had deeper mixing depths and the boreal lake had a shallower mixing depth, suggesting differing patterns in the influence of the timing of ice-out on the length of spring turnover. Differences in nutrient concentrations and dissolved organic carbon between regions and ice-out years were likely driven by changes in precipitation and permafrost thaw. Algal biomass was similar across ice-out years, while cell densities of key Cyclotella sensu lato taxa were strongly linked to thermal structure changes in the Arctic lakes. Our research provides evidence that Arctic and boreal regions differ in lake response in early and late ice-out years, however ultimately a combination of important climate factors such as solar insolation, air temperature, precipitation, and, in the Arctic, permafrost thaw, are key drivers of the observed responses.

The conventional concept of an ‘undifferentiated perianth’, implying that all perianth organs of a flower are alike, obscures the fact that individual perianth organs are sometimes differentiated into sepaloid and petaloid regions, as in the early-divergent angiosperms Nuphar, Nymphaea, and Schisandra. In the waterlilies Nuphar and Nymphaea, sepaloid regions closely coincide with regions of the perianth that were exposed when the flower was in bud, whereas petaloid regions occur in covered regions, suggesting that their development is at least partly controlled by the environment of the developing tepal. Green and colourful areas differ from each other in trichome density and presence of papillae, features that often distinguish sepals and petals. Field experiments to test whether artificial exposure can induce sepalness in the inner tepals showed that development of sepaloid patches is initiated by exposure, at least in the waterlily species examined. Although light is an important environmental cue, other important factors include an absence of surface contact. Our interpretation contradicts the unspoken rule that ‘sepal’ and ‘petal’ must refer to whole organs. We propose a novel theory (the Mosaic theory), in which the distinction between sepalness and petalness evolved early in angiosperm history, but these features were not fixed to particular organs and were primarily environmentally controlled. At a later stage in angiosperm evolution, sepaloid and petaloid characteristics became fixed to whole organs in specific whorls, thus reducing or removing the need for environmental control in favour of fixed developmental control.

Perianth differentiation into distinct morphological whorls (sepals and petals) can be difficult to assess, particularly in early-divergent angiosperms. The perianth of members of Nymphaeales (Cabomba, Brasenia, Barclaya, Euryale, Nuphar, Nymphaea, Victoria, Ondinea) has been described as differentiated into sepals and petals, undifferentiated or presenting examples of both states. In this paper, we review perianth structure in Nymphaeales using morphology and ontogeny in order to determine whether their perianths are dimorphic. The results indicate that although there are differences between the outer and inner perianth series, the organs display few "typical" sepal-petal characteristics. Our observations of the perianth of Nuphar and Nymphaea, and reports on Euryale, show that sepaloid (green) and petaloid (yellow) areas can both occur on individual perianth organs, and that sepaloid areas occur mostly in the regions of the perianth organ that were exposed when the flower was in bud. SEM study revealed further morphological differences between the sepaloid and petaloid areas of Nuphar and Nymphaea and also between the exposed and covered regions of the perianth of Cabomba, Brasenia and Barclaya. We discuss the implications of these results with respect to current concepts of perianth differentiation.