Different class …

Despite my predictions in “Lower forms of pondlife …”, our garden pond is greener now than it was back in April when I last wrote about it.  The frog spawn has hatched into tadpoles but these do not seem to have had the impact that I expected.  If anything, based on a quick visual assessment, the zooplankton which we did see swimming in the open areas are less numerous than earlier in the year.   Maybe the tadpoles are selectively eating these, which might result in less grazing and, as a result, more algae.  However, the green algae in the pond are mostly very small flagellated cells which are extremely hard to photograph, let alone identify. 

Alongside these investigations of the state of the pond in May, I’ve also been looking in more detail at the diatoms I found back in April.  The most abundant diatom turned out to be Nitzschia paleacea.  I had initially thought that this was Fragilaria gracilis, but although the outline and habit were very similar, the chloroplast arrangement, which would have been indicative of Nitzschia rather than Fragilaria, was not easy to see.  The other giveaway that would have led me to Nitzschia rather than Fragilaria is the presence of “fibulae” – silica struts that support the raphe canal.  However, the fibulae on N. paleacea are very fine and, again, quite hard to see on live material.  Some books claim that N. paleacea can also be found in the plankton, with the cells joined at the base to form radial colonies.   However, I did not see any freely-suspended in the samples that I looked at.  They were clustered on the Oedogonium filaments, again, in a very Fragilaria-like manner and, due to their very fine structure the characteristic two chloroplast arrangement was difficult to see.

Nitzschia paleacea from our garden pond, April 2023.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).   The picture at the top shows the pond a month later with water crowfoot in flower.

I normally encounter Nitzschia as single cells moving through biofilms, constantly adjusting their position to maximise their access to light and other resources.  Fragilaria, the genus which I had initially mistaken them for, lacks a raphe (the organelle responsible for movement) but do have an “apical pore plate” at one pole through which mucilage is extruded to enable them to attach.  There is little in the literature, as far as I can see, on how Nitzschia can switch between motile and attached forms.  However, there are reports of planktonic forms where cells aggregate to form stellate colonies.  That, too, implies attachment via the base.   This is a relatively uncommon habit for the genus – only four of 77 species listed in Freshwater Benthic Diatoms of Central Europe have references to the possibility of a planktonic lifestyle, two of which (N. fructicosa, N paleacea) are noted to form stellate colonies.  Interestingly, the N. paleacea I observed growing on our pond’s Oedogonium were all attached separately rather than occurring in clusters whereas I often see epiphytic Fragilaria spp in clusters.   

We need to remember, however, that the strong bias of diatom studies towards benthic habitats and with a focus on cleaned valves means that many of the nuances of the ecology of individual species are missing.  My guess is that expression of these traits is an opportunistic response to local conditions rather than being hardwired into their genome.  It may be that some species are more likely to express these than others, but it is also possible that a few recorded instances have, by dint of being written into a book and, because of the ways in which modern diatomists work, is rarely contradicted by new observations and so becomes dogma.

Oedogonium filaments with epiphytic Nitzschia paleacea from our garden pond, June 2023.  Scale bar: 20 micrometres (= 1/50^th of a millimetre).

The title of this post, incidentally, carries on my predilection for music-based puns.  “Different Class” is, of course, a classic album by Pulp, featuring their most famous song, Common People.   “Class” is also a term that has a taxonomic meaning which meant that the title should work as a way of drawing attention to the identification errors that had dogged my investigation.  However, the arrangement of higher taxonomic groups of diatoms is, itself, still evolving (see “Who do you think you are?”) and the latest system (as used on www.algaebase.com) places both Nitzschia and Fragilaria in the same class, Bacillariophyceae but different subclasses (Bacillariophycidae and Fragilariophycidae respectively).  “Different Subclass” just doesn’t have the same ring to it.

Some other diatoms from our garden pond: a. Hantzschia amphioxys; b. Nitzschia frustulum; c., d. Navicula cryptocephala; e. Gomphonema subclavatum; f. Brachysira microcephala; g. Fragilaria gracilis; h. Tabellaria flocculosa; i. Achnanthidium sp. (girdle view).  Scale bar: 10 micrometres (= 1/100^th of a millimetre).   All were present in trace amounts relative to Nitzschia paleacea.

Some other highlights from this week:

Wrote this whilst listening to: Classic late 60s/early 70s blues band, Groundhogs, following the recent death of their guitarist Tony McPhee.   Also É Soul Cultura volume 2: complilation album by Manchester DJ Luke Una.

Currently reading: Chasing the Monsoon by Alexander Frater, old-school travelogue about India.  Also Small Things Like These, excellent Booker Prize-nominated novella by Claire Keegan

Cultural highlight:  Reality, excellent US film about a whistleblower working in US Intelligence.   Script is taken verbatim from the FBI recording of her interrogation.  The protagonist, incidentally, received a five year prison sentence for exactly the same crime for which Trump has just been charged. 

Culinary highlight:  pork gyros in Newcastle straight after our cinema trip.  Took me straight back to Cyprus …

Lower forms of pond life …

I’m staying close to home for the next post.  Last year we dug a pond in our garden and planted some aquatic and amphibious plants around the margins.  I’ve been intermittently curious about the algae that have colonised the pond but have not actually found the time to put any samples under my microscope until now.   Then, a couple of weeks ago, we had our first sighting of frog spawn in the pond.  When I looked closely at this, I was surprised at how green the water was, bearing in mind the time of year and the scarcity of nutrients (we don’t use any artificial fertilizers in our garden).   I also noticed that the water crowfoot that we added last year had some tangles of green algae already and decided to take a closer look at these. 

Water crowfoot (Ranunculus sp.) in our pond (shown in picture at top of post).  The patch is about 30 cm across.   

I shook a small piece of the Ranunculus in a plastic bottle along with some of the pond water in order to dislodge the attached algae and had a look at what I found under the microscope.   The most abundant, in terms of numbers, was needle-like cells of the diatom Fragilaria gracilis which was attached both to the cells of the filamentous alga (Oedogonium) and, I presume, to the Ranunculus leaves.   There were also a lot of small green algae – probably responsible for the green colour of the water but too finnicky to identify easily.  They will have to wait until another day.

Fragilaria gracilis (top row) and a stalked Gomphonema (bottom) from periphyton associated with Ranunculus and Oedogonium in our garden pond, April 2023.   Scale bar: 10 micrometres (= 1/100^th of a millimetre).  

What attracted my attention, however, was short filaments (“germlings”) of Oedogonium each with a flat circular disc at one end.   I wrote recently about how the green alga Mougeotia stayed attached to the river bed by tangling its filaments amidst mats of a cyanobacterium.   Oedogonium has a slightly more sophisticated option: a dedicated basal cell.   I suspect that these germlings (which arise from zoospores – see “Algae behaving selfishly …”) were originally attached to the leaves of the Ranunculus but had been dislodged by my vigorous agitation.   Some had grown into longer filaments, a few of which had become detached from the Ranunclus plant.   

A series of papers by Jeremy Pickett-Heaps describes this process in detail.   The zoospore (illustrated in my earlier post) is typically egg shaped with a ring of short flagellae near the narrow end (the “dome”).   Pickett-Heaps describes the zoospores elongating, then (literally) shaking off the flagellae.   A number of rhizoids then grow out from the dome and extend across the substratum and secrete material that becomes the holdfast which is, when deposition is complete, a flattened cone anchoring the young filament to a surface.  This series of papers was published in 1972, so must be based on work done a year or so earlier which means that Pickett-Heaps must have had the images of the moon landings in his head when he was thinking about these processes.  I cannot read description of an ungainly zoospore descending to a hitherto unexplored plant surface without Strauss’ Also Sprach Zarathustra* running through my head.

Germlings of Oedogonium each with a flattened basal cell, collected from our garden pond in April 2023.  Scale bar: 20 micrometres.

I’ll finish where I started, pondering the green water of our pond.   There is a mindset amongst some aquatic ecologists and in the media that lots of algae must mean lots of nutrients.   The truth is much more nuanced than that.   The amount of food in your larder is the consequence of how much you put in and how much you take out and the same is true of aquatic habitats.   Our small, still maturing, pond doesn’t have many nutrients going in, but nor are there many invertebrates to eat the algae that do manage to grow.   We’ve also found some frog spawn amongst the fronds of water crowfoot and I’m fairly sure that our “larder” of nutritious algae is soon going to be emptied by hordes of rapacious tadpoles.  

* the iconic theme music for the BBC’s coverage of the Apollo missions.   

Some other highlights from this week: 

Wrote this whilst listening to: new albums by Daughter (Stereo Mind Game) and boygenius (the record)

Currently reading:  Inshallah United: A Story of Faith and Football by Nooruddean Choudry.  

Cultural highlight:  After Impressionism: Inventing Modern Art.  Exhibition at the National Gallery that follows artistic developments either side of Maurice Denis’ 1890 statement that painting was “essentially a flat surface covered with colours assembled in a certain order.”  That was on Monday.  On Friday, I was at the Sage, Gateshead (again) for the Royal Northern Sinfonia, whose programme included Beethoven’s Triple Concerto featuring Sheku Kanu-Mason on cello. Good week.

Culinary highlight:  risotto made with celeriac and blue cheese.

It’s an ecosystem, stupid …

Our garden acquired a small pond earlier this year (see “The story of a pond”) and, five months on, the pond has acquired its first obvious algal growth.   Typing “algae” and “garden pond” into Google returns almost 10 million “hits”, and I’m guessing that the generally negative tone of the top hits will continue all the way down.  The Royal Horticultural Society set the tone with: “Algae can be a major problem in ponds, causing discoloured water, green scum at the pond edges, or dense mats of green growth under the surface. If conditions are favourable, algae will spread quickly and can harm aquatic life”.   Suffolk Wildlife Trust, by contrast, adopt a more balanced attitude: “Algae are important members of a healthy, well-balanced pond ecosystem, providing food for species at the lower end of the food-chain. To deal with algae effectively is not to eradicate it, but to prevent, or control, its excessive growth”.

The flocs in our pond were composed of Oedogonium, a green unbranched filamentous alga often encountered in this blog (see, for example “Under the weather”).   Both the websites I mentioned point to nutrients as a major cause of algae “problems” but the Oedogonium in our pond was covered with needle-like cells of the diatom Fragilaria gracilis, a species that favours nutrient-poor conditions.  Our garden receives no artificial fertiliser so there is little scope for enrichment. 

A more likely reason for the appearance of algal flocs is the warm sunny weather.  Although our pond is shaded, it still receives some direct sunlight, and the water temperature is now over 20^o: ideal conditions for any plant to grow.   We should not really be surprised that a fast-growing organism grows particularly fast when conditions are ideal.   I talked about this in Summertime Blues, describing the effects of the 2018 heatwave on the River Wear.   If climate predictions are correct then we will be experiencing more visible algal growths in the coming years in both ponds and rivers, regardless of how carefully we manage nutrients.   

Flocs of Oedogonium in our garden pond, August 2022.

So what should we do about it?   A garden pond differs from almost every other habitat with which I deal in that one person has complete agency to enact appropriate changes.  Common sense says that a light-touch hands-off approach should suffice, in order to let natural processes determine outcomes.   However, a garden pond is also an extraordinarily small habitat, so there is limited scope for a full range of organisms to create a balanced ecosystem.  The answer is to extend the principles that we apply to the rest of the garden to the pond itself: inspired by large-scale rewilding experiments such as Knepp we’ve adopted a light-touch approach to our lawn (see: “No mow May”).   For this to work, however, we need to acknowledge that a small garden cannot support the large herbivores that would naturally manage vegetation at places such as Knepp, and occasionally mow the grass to simulate their effects.

The same principle applies in a garden pond.   This is not large enough to support a balanced ecosystem where pike predate on smaller fish which, in turn, eat the invertebrates who graze on the algae.   So, instead, we need to act as the primary “grazer”, manually removing the filamentous algae at intervals using a cane or a rake.   They can then be tossed onto the compost heap or used directly as mulches on the flower beds.   The RHS website cautions that “this will only be a temporary solution” but then so is mowing your lawn.   That’s what happens when you create habitats but omit some key ecosystem components.   

Filaments of Oedogonium, along with epiphytic Fragilaria gracilis from our garden pond.   Scale bar: 20 micrometres (= 1/50^th of a millimetre)

The Suffolk Wildlife Trust cautions against manually removing filamentous algae growths as these can harbour larvae of great crested newts but this is more of an issue earlier in the year.   It does bring us full circle, however, reemphasising the point that algae are a natural part of the pond ecosystem.  Learn to love and appreciate them.  Better that than the appalling suggestion on the RHS website that adding dyes to the water will “prevent” algae.   It is an approach that may appeal to antediluvian horticulturists and golf-course groundsmen who still insist on “tidiness” but if you have the tiniest shred of interest in the natural world, you had better remember that algae are the engine rooms of natural aquatic ecosystems and treat them with the respect that they deserve.

P.S. I should add that “we”, when referring to gardening activities, is a euphemism for Heather.  I’m a non-executive director at best in that department. 

Some other highlights from this week:

Wrote this whilst listening to: Sweet Season by the American percussionist Glen Velez.  Also, several Richard Thompson records, following an excellent gig at Whitby Bay Playhouse.   Highlights include I Want to See the Bright Lights Tonight, from 1974, and some very old Fairport Convention songs including Who Knows Where the Time Goes?   It seemed wholly appropriate for a 73-year old to sing this wistful song until you remember that he and his Fairport compatriots wrote and sang these wise-beyond-their years lyrics when they were in their early 20s. 

Currently reading: Reluctant Saint by Donald Spotto, a biography of St Francis ahead of a trip to Asissi in September.   Just finished Delia Owen’s Where the Crawdads Sing.

Cultural highlight:  Brian and Charles: a new British comedy loosely-based on the Frankenstein story which, by coincidence, uses Who Knows Where the Time Goes over the closing credits.

Culinary highlight:  After my eulogy to Casa Mama in the previous post, I discovered St Pancras Square and Coal Drops Yard just behind King’s Cross Station.   Meals at Morty & Bob’s and BAO King’s Cross [ bracketed a meeting in London.