Crustaceans in the Meiobenthos and Plankton of the Thermokarst Lakes and Polygonal Ponds in the Lena River Delta (Northern Yakutia, Russia): Species Composition and Factors Regulating Assemblage Structures

Information about invertebrates in the low-flow water bodies of northeastern Siberia is far from complete. In particular, little is known about crustaceans—one of the main components of meiobenthic and zooplanktonic communities. An open question is which environmental factors significantly affect the crustaceans in different taxonomic and ecological groups? Based on the data collected on the zooplankton and meiobenthos in the tundra ponds in the southern part of the Lena River Delta, analysis of the crustacean taxocene structure was performed. In total, 59 crustacean species and taxa were found. Five of these are new for the region. The species richness was higher in the large thermokarst lakes than in the small water bodies, and the abundance was higher in small polygonal ponds than in the other water bodies. Variations in the Cladocera assemblages were mainly affected by the annual differences in the water temperature; non-harpacticoid copepods were generally determined by hydrochemical factors; and for Harpacticoida, the macrophyte composition was significant. Three types of the crustacean assemblages characteristic of different stages of tundra lake development were distinguished. The hypothesis that the formation of crustacean taxocenes in the Lena River Delta is mainly determined by two types of ecological filters, temperature and local features of the water body, was confirmed.

Growing-Season Temperature Change across Four Decades in an Arctic Tundra Pond

We examined temperature dynamics across a 42-year period in a low-centered tundra polygon pond on the Arctic Coastal Plain of northern Alaska to assess potential changes in thermal dynamics for ponds of this type. Using water temperature data from a pond near Barrow (now Utqiaġvik), Alaska, studied intensively during 1971 – 73 and again in 2007 – 12, we built an empirical model coupling historical air temperatures to measured pond temperatures for four summers. We then used the model to predict summer pond temperatures over a 42-year span, including 1974 – 2008, for which direct aquatic temperature records do not exist. Average pond temperatures during the growing season (1 May through 31 October) increased by 0.5˚C decade-1 or 2.2˚C over the 42-year period. Our simulations predicted the average date of spring thaw for the pond as 2 June (± 3 d), which did not change over the 42-year time period. However, average pond temperature during the first 30 days of the growing season increased from 1971 to 2012, suggesting that recently, ponds are warmer in early spring. The average date of pond sediment freeze over the 42 years shifted later by 15 days, from 28 September in 1971 to 13 October in 2012. These changes correspond to a growing season that has increased in length by 14 days, from 118 days in 1971 to 132 days in 2012. Contemporary temperature measurements in other shallow tundra ponds in northern Alaska show a high degree of temporal coherence (r = 0.93 – 0.99), which warrants the general conclusion that tundra ponds on Alaska’s Arctic Coastal Plain have undergone a significant change in thermal dynamics over the past four decades. Our results provide a means to incorporate these pond types into larger-scale simulations of Arctic climate change.

Pre-emergence growth and development in the arctic midge Trichotanypus alaskensis Brundin

Developing at low mean temperatures, arctic chironomids often have prolonged larval growth yet adult emergence is typically a brief and highly synchronous event. How does a midge population achieve synchronous emergence? Under the Absolute Spring Species Hypothesis (AbSS), adult eclosion by early-emerging species may be synchronized by overwintering as fully mature larvae. Such prepupal larvae would neither feed nor grow after spring thaw, only pupate and emerge. The podonomine Trichotanypus alaskensis Brundin is an abundant midge in tundra ponds on Alaska’s Arctic Coastal Plain, and one of the earliest-emerging species in this chironomid-dominated insect community. T. alaskensis is univoltine in these arctic ponds, with most emergence from any one pond occurring within less than a one-week span during late June, typically about three weeks after pond thaw. We evaluated T. alaskensis for conformity to the AbSS model by documenting the overwintering state of this species in a tundra pond near Barrow, Alaska, then monitoring larval growth and development of the population from spring thaw to pupation. Most T. alaskensis were immature instar IV larvae when collected in late September of both 2010 and 2011, with 10-30% still in late instar III. Immediately after pond thaw in 2011, all collected larvae had imaginal disc primordia showing early stages of instar IV development. Within the first two weeks following pond thaw, most larvae had doubled their dry mass and developed into mature (prepupal) final-instar larvae. Highly synchronized emergence by T. alaskensis is not a consequence of a population overwintering as fully-mature larvae, as per the Absolute Spring Species Hypothesis. Rather, larvae in a given tundra pond appear to develop synchronously throughout the life cycle, including a period of substantial growth and rapid prepupal development between spring thaw and early-summer emergence.

Hydrology drives chemical synchronicity in subarctic tundra ponds

Climate change has implications for the capacity of the many small shallow ponds found at high latitudes to support organisms and store carbon, which in turn has important feedbacks for climate change. As carbon cycling in ponds is linked to nutrient supply, an improved understanding of pond biogeochemistry is needed. Due to logistical challenges, many studies rely on data sets collected during a single campaign that may not be representative of the entire ice-free season. This study characterized spatial and temporal patterns in water chemistry in tundra ponds to determine: (1) if temporal variability in pond chemistry exceeded spatial variability; (2) if temporal variability existed, whether all ponds (or groups of ponds) behaved in a similar temporal pattern, linked to season or hydrology; (3) if spatiotemporal variability in pond biogeochemical signatures could be used to make inferences about processes occurring within ponds and between ponds and surrounding peatlands. Six shallow ponds located in the Hudson Bay Lowlands region were monitored ~ biweekly throughout the ice-free season (May 1–October 31, 2015) for concentrations of nitrogen species and major ions. Temporal variability exceeded spatial variability (variation among ponds) in pond concentrations of most major ions and dissolved organic nitrogen, which appeared to be driven by the evapoconcentration and dilution of pond water. In contrast, the dissolved inorganic nitrogen species were not directly related to pond hydrologic conditions and were instead likely mediated by biological processes within ponds. This work provides an improved understanding of the relative controls of internal and external drivers of pond biogeochemical patterns, and highlights the importance of the highly variable seasonal hydrology. This work also provides insight for future field sampling campaigns.