Mary de Winton

The impact of invasive submerged weed species on seed banks in lake sediments.

Seed banks within submerged sediments were sampled from 21 New Zealand lakes with varying degrees of invasion by vegetatively reproducing, adventive hydrocharitacean species, to examine the influence of vegetation type on seed bank characteristics. Seed number and seed species richness were significantly lower at sites where the submerged vegetation was dominated by the adventive weeds Elodea canadensis Michaux, Egeria densa Planchon or Hydrilla verticillata (Linn. f.) Royle, compared with a predominantly native vegetation. The greater the degree of exclusion of native seed producing plants by adventive weed beds, the greater the apparent impact upon seed abundance. In contrast, seed numbers beneath the adventive weed beds did not differ significantly from sites in lakes where a large scale decline in submerged vegetation had taken place subsequent to hydrocharitacean invasion. The observed impact of weed invasion on lake sediment seed banks is attributed to reduced in situ seed production over time and may be accentuated by increased sediment accumulation beneath tall growing, high biomass weed beds. As seed density influences the speed and extent of germination and plant establishment, our results suggest a large reduction in the re-vegetation potential of invaded lakes. Seed bank decline may also provide an additional ‘ecological feedback mechanism’ contributing to the maintenance of stable, turbid de-vegetated lakes.

Species composition and cyanotoxin production in periphyton mats from three lakes of varying trophic status

In lakes, benthic micro-algae and cyanobacteria (periphyton) can contribute significantly to total primary productivity and provide important food sources for benthic invertebrates. Despite recognition of their importance, few studies have explored the diversity of the algal and cyanobacterial composition of periphyton mats in temperate lakes. In this study, we sampled periphyton from three New Zealand lakes: Tikitapu (oligotrophic), Ōkāreka (mesotrophic) and Rotoiti (eutrophic). Statistical analysis of morphological data showed a clear delineation in community structure among lakes and highlighted the importance of cyanobacteria. Automated rRNA intergenic spacer analysis (ARISA) and 16S rRNA gene clone libraries were used to investigate cyanobacterial diversity. Despite the close geographic proximity of the lakes, cyanobacterial species differed markedly. The 16S rRNA gene sequence analysis identified eight cyanobacterial OTUs. A comparison with other known cyanobacterial sequences in GenBank showed relatively low similarities (91-97%). Cyanotoxin analysis identified nodularin in all mats from Lake Tikitapu. ndaF gene sequences from these samples had very low (≤ 89%) homology to sequences in other known nodularin producers. To our knowledge, this is the first detection of nodularin in a freshwater environment in the absence of Nodularia. Six cyanobacteria species were isolated from Lake Tikitapu mats. None were found to produce nodularin. Five of the species shared low (< 97%) 16S rRNA gene sequence similarities with other cultured cyanobacteria.

Successive macrophyte invasions within the submerged flora of Lake Tarawera, Central North Island, New Zealand

Abstract Lake Tarawera is the lake with the clearest water in New Zealand to be invaded by four particularly successful invasive adventive weeds: Ceratophyllum demersum, Egeria densa, Elodea canadensis, and Lagarosiphon major. E. canadensis was the first to invade the lake followed by L. major which became the dominant tall‐growing species to 6 m depth. Comparisons of lake survey data for macrophytes in Lake Tarawera from 1988 and 1993/94 show that marked changes occurred in the composition and patterns of submerged vegetation as a result of a recent invasion by C. demersum. This species was first recorded in 1988 as infrequent, within a restricted area of the lake. Within 5 years it occurred in 52% of the lake profiles, had a depth range of 0.5–15.5 m, and a median average cover estimated to be between 51 and 75% throughout this depth range. The spread of C. demersum has been at the expense of native vegetation (particularly Characean meadows) and E. canadensis. E. densa was first recorded at the same ti...

Burial limits to the emergence of aquatic plant propagules

Abstract The sediment depth limits to germling emergence were determined for five common New Zealand submerged plants. A significant trend was observed of declining emergence with increasing burial depth. Emergence from small oospores (13–16 μg) of Nitella pseudoflabellata A. Br. and Nitella leptostachys A. Br. was limited to the surface sediment (<25 mm). Potamogeton ochreatus Raoul seed (1460 (μg) emerged from <50 mm depth. Chara corallina Willd. oospores (148 μg) emerged from burial depths of at least 50–75 mm, while Chara globularis Thuill. oospores (50 (μg) were able to emerge from the maximum tested depth of 100 mm. The observation that burial limits to emergence by Chara species were at least twice that of the much larger P. ochreatus seed is at odds with theory that the size of propagule reserves act to determine emergence ability. However, it is in keeping with suggested energy efficiency of shoot extension in the macroalgae versus vascular species. Results indicate that assessment of submerged seed banks should concentrate on the upper 50 mm of sediment which contains most “ecologically active” propagules.

Fish effects on charophyte establishment in a shallow, eutrophic New Zealand lake

Abstract Fish exclosures were deployed within shallow, eutrophic Lake Rotoroa, Hamilton, New Zealand to assess the influence of an alien‐dominated fish population on establishment by charophytes. Replicated mesh exclosures were placed at sites differing in wave exposure and received transplanted charophyte oospores and plants, with identical material placed outside. Charophyte establishment from oospores and plant biomass were greater inside exclosures compared with outside. Wave exposure did not depress germling response and the effect of fish exclosures on charophyte establishment was least apparent at the most exposed site. Exclosures did not have a statistically significant influence on the light climate, and epiphytic algal development was similar or higher inside the exclosures. However, sediment mobilisation was lower inside the exclosures. Results suggest that fish were primarily responsible for the poor performance of unprotected charophytes in Lake Rotoroa, with fish effects on plants operating via direct disturbance or grazing.

SUBMERGED VEGETATION OF LAKES TE ANAU, MANAPOURI, MONOWAI, HAUROKO, AND POTERITERI, FIORDLAND, NEW ZEALAND

Abstract The submerged vegetation in Lakes Te Anau, Manapouri, Monowai, Hauroko, and Poteriteri from the South Island of New Zealand, was surveyed in 1993 using SCUBA methods to record a range of botanical parameters to the deepest extent of the vegetation. This is the first detailed account of the submerged vegetation in these lakes and the first description for Lake Poteriteri. The vegetation of these lakes is comprised predominantly of native species. Isoetes kirkii dominates the shallow‐water assemblage in all lakes. Hydatella inconspicua, an endemic shallow‐water plant of rare status, is reported for the first time in the South Island, New Zealand. Formerly only reported from a limited number of North Island sand dune lakes, the discovery of these southern disjunct populations of H. inconspicua means that this species should no longer be considered endangered. The tall‐growing native vascular species, Potamogeton cheesemanii, Myriophyllum propinquum, and M. triphyllum were recorded in all five lakes....