Samples of planktonic algae from major lakes in the English Lake Districthave been investigated over many years as part of a long-termmonitoring programme set up by the FBA and continued by the IFE.Some lakes have been sampled regularly for long periods, others lessfrequently or only occasionally. Windermere north and south basins, andthe neighbouring lakes Blelham Tarn and Esthwaite Water, have beenunder observation, mostly on a weekly basis, since the programmebegan in 1945. During this time diatoms have played a significant role inthe population dynamics of these lakes (Lund 1954 and unpublisheddata; Tailing et al. 1986)

Elizabeth Y. Haworth

Hilda M. Canter

FBA Journal System (Freshwater Biological Association)

TWO PLANKTONIC DIATOMS 39 
THE OCCURRENCE OF TWO NEW PLANKTONIC DIATOM 
POPULATIONS IN THE ENGLISH LAKE DISTRICT: 
AULACOSEIRA ISLANDICA SUBSPECIES HELVETICA AND 
A. AMBIGUA 
HILDA M. CANTER AND ELIZABETH Y. HAWORTH 
(Dr H. M. Canter, Freshwater Biological Association 
and Dr E. Y. Haworth, Institute of Freshwater Ecology, 
The Ferry House, Far Sawrey, Ambleside, Cumbria LA22 OLP, England.) 
Introduction 
Samples of planktonic algae from major lakes in the English Lake District 
have been investigated over many years as part of a long-term 
monitoring programme set up by the FBA and continued by the IFE. 
Some lakes have been sampled regularly for long periods, others less 
frequently or only occasionally. Windermere north and south basins, and 
the neighbouring lakes Blelham Tarn and Esthwaite Water, have been 
under observation, mostly on a weekly basis, since the programme 
began in 1945. During this time diatoms have played a significant role in 
the population dynamics of these lakes (Lund 1954 and unpublished 
data; Tailing et al. 1986). 
There have been very few published surveys of algal distribution in all 
the other large and small lakes or tarns in the English Lake District; 
Knudson (1954) collected material for a detailed study of the diatom 
genus Tabellaria and Lind (1980) studied desmids. In 1983 and 1984, 
samples of diatoms were collected from surface sediments of almost all 
the standing waters in the county of Cumbria, including the Lake District 
lakes (Haworth et al. 1988). One result was a review of all the Melosira 
taxa (now mainly transferred to the genus Aulacoseira) present in these 
waters at that time (Haworth 1988). Whilst several taxa are prominent in 
acidic lakes and tarns, A. subarctica (O. Miiller) Haworth is the most 
abundant species in the larger and more productive waters. This taxon, 
present throughout the post-glacial sediment profile of Windermere 
(Pennington 1943), was first noted in the plankton there in 1907 (West & 
West 1909) but it was misidentified as M. granulata Ehrenberg. The 
recent increases in abundance of this taxon led to an extensive study to 
explain its success (Lund 1954). However, in a footnote added in proof, 
Haworth (1988) revealed that a new taxon, A. islandica subspecies 
helvetica (O. Muller) Simonsen, had just been seen in the phytoplankton 
of Windermere. Recently a new study of Bassenthwaite Lake has shown 
that another Aulacoseira taxon, A. ambigua (Grunow) Simonsen, has 
now appeared there. 
40
 H. M. CANTER AND E. Y. HAWORTH 
TWO PLANKTONIC DIATOMS 41 
42 H. M. CANTER AND E. Y. HAWORTH 
Aulacoseira islandica subspecies helvetica in Windermere 
Occasional filaments of Aulacoseira islandica subspecies helvetica (syn. 
Melosira islandica subspecies helvetica O. Muller and hereafter referred 
to as helvetica ) were first noted in the plankton of the south basin on 27 
October 1987. Specimens were then seen during April and May 1988, 
from October 1988 to May 1989, and September 1989 to May 1990. In 
June 1988 a few valves of helvetica were also found in the top centimetre 
of a lake sediment core (Haworth, unpublished). By the spring of 1989, it 
was already clear that helvetica had begun to thrive in Windermere and 
by April 1990 it had increased so much that it was more abundant than 
A. subarctica (syn. Melosira italica subspecies subarctica O. Muller). It 
was first noted in the north basin of Windermere in the spring of 1988 
and, although less numerous than in the south basin, numbers have 
steadily increased. Unpublished data (C. Butterwick, pers. comm.) shows 
that both species of Aulacoseira had reached their peak by 3 April 1990. 
The respective numbers of cells present in the south basin at that time 
were 643 ml"1 for A. subarctica and 1273 ml"1 for helvetica. 
Filaments of helvetica are composed of cells containing up to 10 
discoid plastids (Figs 1, 2) and the pyrenoids, as noted by Skuja (1948), 
show up well in cells stained with iodine (Fig. 2b arrowed). The silica 
cell-walls are thinner than those of A. subarctica (Figs 3, 8) and lack an 
especially thickened pseudoseptum. The areolae are arranged in straight 
longitudinal rows on the mantle (Figs 4, 5). Scanning electron microscope 
(SEM) micrographs show that the areolar pattern in helvetica consists of 
fine pores (Fig. 5) and the frustules are linked together by many small 
spathulate spines (Fig. 6) which are not able to slide apart in the same 
manner as the pointed spines of A. subarctica. As no separation valves 
have been noted in helvetica, filaments usually break by the disruption of 
a cell so that one frequently finds empty valves at the ends, often 
terminated by the first girdle-band (the valvocopula; Fig. 5). Several 
rimoportulae are visible on the valve mantle (Fig. 5 arrowed). 
In comparison, live filaments of A. subarctica are readily identified by 
the thickness of the silica walls, especially the thickened area of the 
pseudoseptum (sulcus) (Fig. 8), the spiral lines of areolae on the mantle 
(Figs 9, 10), the shape and number of the plastids (up to 4, Fig. 7) and, 
above all, the ring of large equal-sized spines visible not only at the end 
of the filaments but often overlying the adjacent cells (Figs 9, 10). Under 
the SEM, the spines are seen to be straight, tapering from the base, and 
they fit into grooves in the sibling valve in a manner that allows cells to 
slide apart freely. Lines of areolae end both at the base of the spine and in 
the spine groove (Fig. 10 and Haworth 1988). 
TWO PLANKTONIC DIATOMS 43 
Aulacoseira ambigua in Bassenthwaite Lake 
In the autumn of 1990, phytoplankton samples were collected from 
Bassenthwaite Lake, near Keswick. These included a high proportion of 
Aulacoseira filaments which differed from those of A. subarctica, which is 
normally found there together with Fragilaria crotonensis , Asterionella 
formosa and Cyclotella pseudostelligera. The taxon (Figs 11-16), which 
now seems to have replaced A. subarctica, has been identified as 
Aulacoseira ambigua (syn. Melosira ambigua Grunow). Although the 
phytop lankton in Bassenthwaite Lake has only been examined 
infrequently, this species has not been observed here before, in either 
plankton or surface sediments analysis. Phytoplankton collected by R. 
Mubamba in 1987 and identified only to genus (Tailing et al. 1989) has 
now been re-examined and while spring samples include A. subarctica, 
the autumn samples contain several filaments of A. ambigua as well. As 
inclusion of Melosira ambigua in the species list for Grasmere (Reynolds & 
Lund 1988) refers to just a few dead cells, Bassenthwaite Lake represents 
another new record for the English Lake District and the rest of Cumbria. 
Again, live filaments look different from those of A. subarctica; the cell 
walls are not as thick and the plastids are more numerous (Fig. 11). At high 
magnification one can often observe the hollow sulcus that is typical for 
this taxon (Fig. 12 arrowed). The areolae are spirally arranged on the 
mantle but are larger and more quadrate than those on A. subarctica (Figs 
13, 14, 16) and there is usually one rimoportule visible close to the sulcus. 
A. ambigua is one of the species of Aulacoseira where terminal and 
middle cells differ greatly in their modes of linkage; most cells are held 
together by very tiny interlocking spines which are triangular with 
crescentic edges (Fig. 15), and are only separated by breakage; only rarely 
were end-valves observed with small, regularly-pointed spines. 
Where have these diatoms come from? 
Two Aulacoseira taxa appear to have invaded two different lakes in the 
English Lake District within the last few years. They have probably come 
from outside Cumbria, for neither taxon was observed in the surface 
sediment assemblages from any of the local lakes and tarns during the 
surveys made in 1983 and 1984 (Haworth 1988). Neither taxon is at all 
frequent in northern Britain, although helvetica has been recorded in 
North Wales (Woodhead & Tweed 1954-55) and the Shetland Isles (Carter 
& Bailey-Watts 1980), and is known from some Irish loughs (Gibson et al. 
1980). It is frequently observed in eutrophic lakes in Scandinavia (Nygaard 
1956; Molder & Tynni 1967) but was first found in Zi ir ichsee in 
1905 (Mul ler 1906). Hustedt (1945) considered it to be 
indicative of meso- to eutrophic lakes which are less productive than those 
44 H. M. CANTER AND E. Y. HAWORTH 
containing A. granulata (Ehrenberg) Simonsen; the latter has not yet been 
recorded from Cumbrian lakes. Occasional specimens of the species itself, 
A. islandica (O. Miiller) Simonsen, have been found in the earliest post-
glacial sediments (age c. 9000 BP) of Windermere (Haworth, unpublished 
records). It is possible, but unlikely, that tiny populations continued to 
exist throughout the post-glacial despite unsuitable o l igotrophic 
conditions. It is however a fact that neither A. islandica nor its subspecies 
helvetica had been seen in the several thousand algal samples which have 
been closely inspected over the last c. 50 years. 
A. ambigua occurs in many water-bodies in England, in the Shropshire-
Cheshire meres and in East Anglia; in the latter area it was widespread and 
abundant in the 1950s (Lund, unpublished data) and has been found in a 
recent survey (K. B. Clarke, pers. comm.). The only record from northern 
Britain is from a reservoir in Northumberland (J. R. Carter, pers. comm.). 
This diatom is also a recognized indicator of eutrophic lakes. 
In both instances there are no known local sources of these two new 
taxa; the nearest recorded site of helvetica is over 160 km (c. 100 miles) 
distant, while that for A. ambigua is either 95 km (c. 60 miles) away in 
Northumberland or 145 km (c. 90 miles) away in the Cheshire-Shropshire 
meres. 
Any new water-body is rapidly colonized by algae and a plankton 
readily develops in new reservoirs, e.g. Cow Green (Atkinson 1988), even 
though there are no suitable sources in the catchment. New taxa also 
arrive in established lakes, albeit at a slower rate since they must compete 
with resident populations. Although algae are obviously transported 
passively (Tailing 1951), there are no detailed studies showing how far 
viable cells or spores can travel. Algae are found in air samples but are 
they viable? Birds and animals are known to carry material on their bodies 
and in their guts, and diatoms have been found and cultured from faecal 
material from ducks (Holmes & Croll 1984; Atkinson 1972). Nowadays 
humans also provide a likely means for their dispersal; increasingly, lakes 
are used for recreational, educational and other purposes. Such pursuits 
may readily lead to the transfer of diatoms and other biota from lake to 
lake. This has serious implications for the spread of algal blooms and other 
nuisance organisms in amenity waters and for the safety of some 
endangered or rare species, especial ly as these movements are 
unpredictable. 
FIGS 11-1.6. (Opposite). Aulacoseira ambigua (Grunow) Simonsen. 1 1 , arrangement of 
plastids in the cells (x800). 12, median view of cells and valve showing the hollow sulcus (-*) 
(x2000). 13 and 14, spiral arrangement of quadrate-shaped areolae, with rimportula (-») 
adjoining the sulcus (x2000 and x4000). 15 and 16, SEM micrographs (x22000 and x9000 of 
linking (15) and terminal (16) spines. 
TWO PLANKTONIC DIATOMS 45 
46 H. M. CANTER AND E. Y. HAWORTH 
Are these taxa indicative of rising nutrient levels? 
Ecological information suggests that they are. Certain lakes in the English 
Lake District have been markedly enriched in the last 30 years due to 
increasing sewage input, use of detergents and, to a lesser extent, 
agricultural fertilizers (Lund & Moss 1990). Windermere has certainly 
been altered (Tailing et al. 1986) and Bassenthwaite Lake is changing too 
(Tailing et al. 1989). 
It has already been noted that organisms continually arrive in new 
areas but they can only invade successfully when nutrient levels are 
suitable and a niche is available. New algae have been seen before in 
Windermere as well as in other lakes. It is possible that these two 
diatoms have been able to compete with the established populations 
because they grow better in eutrophic waters; their presence may 
indicate a plant succession due to increased nutrient availability. Clearly 
it takes several years for visible diatom populations to arise from the 
original inoculum, and the last few mild winters and warm summers 
have therefore merely enhanced very small and unseen populations that 
were already present. 
It is too soon to say if these Aulacoseira taxa wil l thrive and if they wil l 
totally replace the more usual A. subarctica or coexist. Much wil l depend 
on the nutrient levels sustained in the lakes. In Windermere the main 
cause of enrichment has been the phosphate in sewage effluent. This 
input is now being reduced by North West Water with alterations already 
made to the Ambleside sewage works and soon to be working at the 
Tower Wood works. Reduced nutrients may reverse the current trend 
from A. subarctica to A. islandica subspecies helvetica or A. ambigua, or 
may lead to other changes. Changes may also be taking place elsewhere; 
there is therefore a general need for a continued programme of long-term 
monitor ing. Where records are insufficient it is always possible to 
interpret past changes, using the diatom remains accumulated in the 
sediments of the lakes. 
Acknowledgements 
The authors wish to thank those who have provided us with samples, 
also C. Butterwick, for information on cell concentrations, and J. R. 
Carter for information from his extensive collections. We are especially 
grateful to J. W. G. Lund for information from his unpublished records 
and for advice on the manuscript. Access to the Fritsch Collection of 
Algal Illustrations was also essential. This article is dedicated to all those 
who have monitored the phytoplankton of these lakes. 
TWO PLANKTONIC DIATOMS 47 
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The occurrence of two new plankton diatom populations in the English Lake District:  Aulacoseira islandica

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The occurrence of two new plankton diatom populations in the English Lake District: Aulacoseira islandica

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