Cathy Kilroy

Historical processes constrain patterns in global diatom diversity

There is a long-standing belief that microbial organisms have unlimited dispersal capabilities, are therefore ubiquitous, and show weak or absent latitudinal diversity gradients. In contrast, using a global freshwater diatom data set, we show that latitudinal gradients in local and regional genus richness are present and highly asymmetric between both hemispheres. Patterns in regional richness are explained by the degree of isolation of lake districts, while the number of locally coexisting diatom genera is highly constrained by the size of the regional diatom pool, habitat availability, and the connectivity between habitats within lake districts. At regional to global scales, historical factors explain significantly more of the observed geographic patterns in genus richness than do contemporary environmental conditions. Together, these results stress the importance of dispersal and migration in structuring diatom communities at regional to global scales. Our results are consistent with predictions from the theory of island biogeography and metacommunity concepts and likely underlie the strong provinciality and endemism observed in the relatively isolated diatom floras in the Southern Hemisphere.

ENVIRONMENTAL CONTROL OF STALK LENGTH IN THE BLOOM-FORMING, FRESHWATER BENTHIC DIATOM DIDYMOSPHENIA GEMINATA (BACILLARIOPHYCEAE)(1).

Blooms of the freshwater stalked diatom Didymosphenia geminata (Lyngb.) M. Schmidt in A. Schmidt typically occur in oligotrophic, unshaded streams and rivers. Observations that proliferations comprise primarily stalk material composed of extracellular polymeric substances (EPS) led us to ask whether or not the production of excessive EPS is favored under nutrient‐limited, high‐light conditions. We conducted experiments in outdoor flumes colonized by D. geminata using water from the oligotrophic, D. geminata–affected Waitaki River, South Island, New Zealand, to determine the relationship between D. geminata stalk length, cell division rates, and light intensity under ambient and nutrient‐enriched conditions. Stalk lengths were measured in situ, and cell division rates were estimated as the frequency of dividing cells (FDC). FDC responded positively to increasing light intensity and to nutrient additions (N+P and P). Under ambient conditions, stalk length increased as light level increased except at low ambient light levels and temperature. Nutrient enrichment resulted in decreased stalk length and negative correlations with FDC, with this effect most evident under high light. Our results are consistent with the hypothesis that extensive stalk production in D. geminata occurs when cell division rates are nutrient limited and light levels are high. Thus, photosynthetically driven EPS production in the form of stalks, under nutrient‐limited conditions, may explain the development of very high biomass in this species in oligotrophic rivers. The responses of FDC and stalk length under nutrient‐replete conditions are also consistent with occurrences of D. geminata as a nondominant component of mixed periphyton communities in high‐nutrient streams.

Eunophora gen. nov. (Bacillariophyta) from Tasmania and New Zealand: description and comparison with Eunotia and amphoroid diatoms.

A new raphid pennate diatom genus, Eunophora, and three new species are described from highland lakes and streams in Tasmania and New Zealand. Eunophora tasmanica and E. indistincta are only found in Tasmania; E. oberonica and a fourth species (Eunophora sp. 1) also occur in New Zealand. The presence of polar rimoportulae in E. tasmanica and E. oberonica, the relatively simple structure and arrangement of the raphe system and the stria pattern indicate that Eunophora belongs to the subclass Eunotiophycidae. However, it differs from the other genera in this subclass in the amphoroid symmetry of the cells, the length of the raphe slits and the non-coaxial internal central raphe endings; also unusual are the position of the raphe on the valve face instead of on the ventral mantle and the presence of many small discoid or band-like chloroplasts. Eunophora may represent a link between the Eunotiophycidae and the amphoroid genera of the Bacillariophycidae. Eunophora is characteristic of dystrophic to (ultra-)ol...

The Didymo story: the role of low dissolved phosphorus in the formation of Didymosphenia geminata blooms

We outline, in chronological sequence, the events and findings over 25 years that have shaped our understanding of Didymosphenia geminata (Lyngbye) M. Schmidt blooms. Starting with the first appearance of D. geminata mats in streams on Vancouver Island in the late 1980s and followed years later by blooms in Iceland, South Dakota and Poland, D. geminata blooms were enigmatic for nearly 20 years. Early papers exploring whether blooms were caused by environmental change consistently failed to identify any specific factor(s) associated with their onset. Following the D. geminata outbreak in New Zealand in 2004 that seemed to result from an introduction of the species, the possibility that blooms that had previously occurred elsewhere in the world might also be explained by the introduction and movement among watersheds of a new variant with a bloom-forming tendency was touted and widely accepted. Now, however, the identification of very low soluble reactive phosphorus (SRP; below ∼2 ppb) as the proximate cause of bloom formation, has led to the more likely explanation that D. geminata blooms are the result of large-scale human intervention in climatic, atmospheric and edaphic processes that favour this ultra-oligotrophic species. In this new view, blooms of D. geminata are not simply due to the introduction of cells into new areas. Rather, bloom formation occurs when the SRP concentration is low, or is reduced to low levels by the process of oligotrophication. Mechanisms that potentially cause oligotrophication on global and regional scales are identified.

Benthic Diatom Communities in Subalpine Pools in New Zealand: Relationships to Environmental Variables

In spite of their potential use as indicators of both present and past environmental conditions, little is known about the diatom communities in the many small water bodies at high altitudes in New Zealand. We sampled benthic diatoms at 20 sites in a typical subalpine mire pool/tarn complex near Arthur’s Pass in South Island, New Zealand in the austral spring 2001. The aims were to characterise the diatom communities, including identification of a possible endemic component, and to investigate relationships with environmental variables. The community at genus level was consistent with the peat-bog diatom flora reported from elsewhere except for the common occurrence of the Tasmania/New Zealand endemic genus Eunophora. At the species level, 27 of the 52 most common taxa appear to correspond to known species from the Northern Hemisphere and are therefore presumed to be cosmopolitan in their distribution. Just two taxa are known from the Southern Hemisphere only, however identification of the remaining common species proved problematic. Analysis using the BIO-ENV procedure of the PRIMER computer program confirmed an expected strong association between diatom community composition and pH, with water conductivity and gilvin also important. Weighted averaging regression and cross-validation using C2 software enabled selection of four diatom species as potentially sensitive indicators of certain pH levels. Neither species of Eunophora showed a strong preference for pH or for any of the other environmental variables measured, indicating that other factors are determining their distributions. The strength of the species–environment relationships found in this small survey suggests good potential for monitoring current conditions and for palaeoecological applications. Extension of the dataset with information from other alpine/subalpine areas is desirable, as is the compilation of a regional diatom identification guide for these habitats.

New species of Fragilariforma (Bacillariophyceae) from New Zealand and Australia

Abstract Two new species of Fragilariforma, F. cassieae sp. nov. and F. rakiuriensis sp. nov., are described from material from New Zealand, with reference to populations found in Australia. The new species are similar in shape and size to Fragilariforma lata and F. hungarica var. tumida but are separated on the basis of areola and stria density, and shape. The distinguishing features of F. cassieae are a rhombic shape with narrow protracted ends, and striae with prominent areolae. In F. rakiuriensis, small forms are similar in shape to F. cassieae, but larger cells are elongated with a constriction at the centre. The striae are more widely spaced than in F. cassieae with a slightly irregular appearance and fine, closely spaced areolae. We discuss the relationships of the new species within the genus Fragilariforma, and also discuss the possible existence of further related taxa in the material examined, or marked shifts in form in their morphological progression during cell division. The two species described represent further evidence of a higher degree of endemism in Southern Hemisphere freshwater diatoms, and particularly in New Zealand and Australia, than has previously been assumed.

Use of the SHMAK clarity tube for measuring water clarity: comparison with the black disk method

Abstract A metre‐long transparent tube for measuring visual water clarity was developed as part of the New Zealand Stream Health Monitoring and Assessment Kit (SHMAK) for use by non‐scientists. Water clarity measured using the tube (the horizontal sighting range of a black target) was compared with that measured using the standard “black disk” method (in which a black target is viewed horizontally underwater) to determine the feasibility of estimating black disk clarities from clarity tube readings. Readings using the two methods showed good agreement for waters with clarity <c. 50 cm. In clearer waters the slope of the relationship depends on the reflectivity of the background material used in the clarity tube. With a reflective background (white, reflectance c. 75%; or grey, reflectance c. 55%), the clarity tube readings are directly proportional and approximately equal to the black disk readings. With a black background (reflectance c. 5.5%), clarity tube readings >c. 50 cm are lower than the black disk visibility and follow a logarithmic relationship with visibility, which enables useful readings to be made in clearer water. Independent testing of the relationship showed that black disk visibilities may be predicted with reasonable confidence in waters that contain very low concentrations of dissolved humic materials (yellow substance). In waters that are noticeably brown‐coloured, the relationship is less consistent especially at black disk visibilities of more than 1.5 m. Recommendations are given for the use of the clarity tube.

Attachment and short-term stalk development of Didymosphenia geminata: effects of light, temperature and nutrients

We measured cell attachments and stalk lengths of the stalked diatom Didymosphenia geminata in situ, using in vivo staining, under different nutrient (phosphorus and nitrogen) and light treatments. Our aims were: (1) to investigate the effects of light and temperature (season) on attachment and stalk growth, including the effect of ultraviolet radiation (UVR), which has been suggested as a possible factor favouring D. geminata proliferation; and (2) to test the hypothesis that enrichment with dissolved phosphorus (DRP) inhibits the initial attachment of D. geminata cells. Although low concentrations (below ∼ 2 mg m−3) of DRP appear to be responsible for D. geminata proliferation, its general absence where DRP>2 mg m−3 is not understood. The experiments were conducted in outdoor channels over 14 months, spanning a range of water temperatures and light intensities. Increasing visible light intensity usually had a positive effect on attachment densities and stalk length, but both were depressed by very high intensities. Exposure to UVR generally led to lower attachment rates and shorter stalks. Increasing water temperature with season had a positive influence on the proportion of cell attachments producing stalks. Elevated nutrients (up to 6.6 mg m−3 P, and up to 115 mg m−3 N above ambient) did not affect D. geminata cell attachment unless the treatment channels contained previously colonised substrata. Nutrient enrichment negatively affected stalk length. Earlier findings of a positive role of light and a negative effect of nutrients on stalk length in D. geminata were corroborated, except that photoinhibition was demonstrated at very high visible light intensities. There was no evidence of a positive effect of UVR on D. geminata proliferation at the initial stages of attachment and growth. The results indicated that the absence of D. geminata from rivers with high DRP concentrations is not the result of nutrient interference with initial cell attachment.

Estimating periphyton standing crop in streams: a comparison of chlorophyll a sampling and visual assessments

Abstract Periphyton standing crop is often measured as chlorophyll a but there is increasing interest in using visual assessments of periphyton cover. Visual methods are rapid and require no sample analysis, but can be regarded as subjective with high inter-operator variability. We investigated variability in periphyton standing crop across operators, rivers and time, as determined visually and from chlorophyll a measurements. We found that visual assessments (comprising percentage cover estimates of up to eight periphyton categories) distinguished sites and occasions as effectively as chlorophyll a. Furthermore, an estimate of chlorophyll a could be derived from the visual assessments. Because our surveys were conducted in only three rivers, general applicability of the derivation of estimated chlorophyll a from the visual assessments warrants further investigation. The current recommendation of 20 views was sufficient for realistic visual assessments of average cover. Overall, our results indicated that inter-operator variability in visual assessments need not be a major concern, given adequate training.

Laboratory study of the survival and attachment of Didymosphenia geminata (Bacillariophyceae) in water sourced from rivers throughout New Zealand

Abstract: Didymosphenia geminata is a freshwater diatom that can form extensive nuisance mats in rivers and streams. First detected in New Zealand in 2004, D. geminata has been found in over 150 rivers throughout the South Island. The diatom exhibited interesting distribution patterns in New Zealand, where it has remained absent from several apparently suitable habitats. The aim of this study was to investigate the role of New Zealand river water chemistry in the establishment of D. geminata. Water was collected from 16 North Island and four South Island rivers, and four South Island groundwater-fed creeks where D. geminata was absent, and from seven South Island rivers with low to medium levels of D. geminata and six South Island rivers with high levels of D. geminata. Water chemistry parameters were measured for these sites. The survival and attachment of D. geminata cells to the surface of culture plates were monitored in the laboratory under controlled conditions. Cells survived, attached and underwent cell division in waters with a wide range of chemistries, including water where D. geminata was absent. No correlation was found between cell survival, attachment, or division and any of the individual elements, compounds, or nutrient ratios tested. Findings in this study suggested that water chemistry, in isolation of other environmental parameters, was unlikely to be the sole variable responsible for D. geminata distribution in New Zealand. The attachment of free D. geminata cells to a substrate was a prerequisite for cell division, indicating that substrate composition may play an important role in D. geminata establishment.