Summer of 2004 satellite images of Manitoba’s Great Lakes

MODIS images

The satellite images on this page were recorded by the Moderate Resolution Imaging Spectroradiometer (MODIS) now orbiting aboard two of NASA’s satellites, Terra and Aqua. Unless otherwise noted, the actual colour renditions were prepared by the MODIS Rapid Response Team out of the University of Maryland who provide near-real time colour composite images on their web site http://rapidfire.sci.gsfc.nasa.gov/realtime/ -- except that I have changed the tone curve to emphasize colour differences in the lake, at the expense of brighter areas like clouds.

Click on each underlined date below to view a larger copy of the image.

18:20 1 December 2004 19:10 2 December 2004

Between the 26^th of November and the 1^st of December, Lake Winnipeg appears to have frozen over, from only shallow bays covered by ice on the 26^th to only a narrow lead along the west shore of the North Basin on the 1^st. The Environment Canada record at Berens River, halfway down the lake, indicates continuous sub-zero temperatures since the 23^rd, but it wouldn’t surprise me if most of the cover formed in the one night of the 30^th to the 1^st, when Berens reported -18 to -22^oC in a period of perfect calm from 8 p.m to 4 a.m. That quick freeze-up over the whole lake is quite a contrast to last year, when the South Basin was mostly ice-covered by mid-November and the North Basin took another couple of weeks to freeze over.

19:25 26 November 2004 19:30 28 November 2004

Ice cover looks pretty solid over the shallow bays, and has started to form in the South Basin – a good example of how both basins tend to freeze over at much the same time of year. Ice has spread quickly over much of the western North Basin in the last two days. Because the shallower South Basin cools more quickly than the North (which has a lot more summer-stored heat to lose) ice cover develops on the two basins through much the same period in spite of the considerable north-south temperature gradient. Spring break-up is often more spread out. It tends to be over in the South Basin a couple of weeks earlier than in the North Basin. See for instance, 12:00 18 May 2004.

Note that the colours in these images have been adjusted differently compared to images from earlier this year – trying to deal with the high contrast and still discriminate colours in the lake. As a result, the intensity of greens and tans is exaggerated and their relationship with water quality parameters is not quantitatively the same as in the ealier images.

19:30 12 November 2004 17:35 14 November 2004

Remarkably, the turbid plume created during the strong winds of early October is still visible in the central North Basin. But freeze-up is fast approaching. (There’s already been ice on & off & on again on the Assiniboine River down here in Winnipeg.) There’s snow on the ground east of the Narrows – it would be harder to judge whether the white that you can see there might be clouds – but it covers the same patches in both images, two days apart. Ice covers many of the lakes near the north end of Lake Winnipeg, as well as Limestone Bay on Lake Winnipeg itself (left image). To some extent you can judge the relative depth of the lakes to the northwest of Lake Winnipeg by the timing of freeze-up. The ice-free lakes in this image are deeper than the ones with ice cover – there is simply more water in them to be cooled before freezing can begin. In the Outlet Lakes along the path of the Nelson River out of Lake Winnipeg, the timing is dominated by a different process – the older ice (bright white – probably with a cover of new snow) is on the lakes and bays off the paths taken by the slightly warmer Lake Winnipeg water through Playgreen and Kiskittogisu Lakes. I can’t decide whether the pale green along the path of flow through these lakes is very turbid water, or thin new ice before snow has brightened it.

You can also see the turbid plume of the Red River in the South Basin (clearest on the image on the right). All last week, the Red was flowing high with runoff from heavy rain in North Dakota and Minnesota. As it has been all summer, the Winnipeg River plume is strong, too – but dark, without a heavy sediment load scattering light back upwards.

19:15 6 October 2004

27, 30 September & 6 October 2004 The North Basin was buffeted by strong winds for four days last week – the 1^st through the 4^th of October. We sat through the storm – including rain, snow and sleet – in the little harbour at George’s Island (red circle above; see pictures below). The scenes above show the effect on the North Basin. The 30^th was near calm, and a surface bloom developed. You can see it most strongly in the northeast corner of the lake. Down below, I’ve posted a couple of pictures taken from the Namao when we passed through the thickest part of the bloom that afternoon (red line in the centre image). The red line in the lower image shows the Namao’s path on the 5^th of October. All that day we passed through water that looked as turbid as the water in the South Basin. The light tan colour stretching up through the centre of the North Basin past Long Point – a turbid plume stretching at least 80 km further to the north than it had before the storm -- must be sediment-laden water from the south that was carried northwards as the water swashed back after being driven south by the winds. Check out the water level graphs I copied from the Water Survey page, below – showing large water level changes at the north and south end that imply an enormous volume of water was moved in the swashes south and then back north, twice during the storm. The alternative – that it is sediments stirred up from the bottom – is unlikely, since it is over the deepest water, more than 16 m deep. Moreover, the plume is probably a fairly shallow surface feature – warmer, and therefore less dense water washing back from the south over the surface of the colder northern water. In that case, we may see it gradually thin out as it gets mixed with clearer water from below over the next few weeks. Of course, there are but a few weeks left before freeze-up begins.

That was a strong blow. It peaked at a mean hourly speed of 82 km/h measured at the Georges Island station. (We know what if felt like. We were out there watching the anemometer spin, the Namao safely tucked in harbour.) During the days with the strongest north winds, the water level twice dropped by more than half a metre at Montreal Point, near the north end of the lake. At the same time, it rose by nearly 0.8 m at Pine Dock, in the Narrows. I’ve aligned the graphs and marked the peaks at Pine Dock with vertical lines. Both occur a few hours after the peak northerly winds. You can access water level and flow records for many of the Water Survey of Canada’s stations at http://www.wsc.ec.gc.ca/products/main_e.cfm?cname=products_e.cfm. The weather data are from the Atmospheric Environment Service site at http://weatheroffice.ec.gc.ca/canada_e.html where links lead to real-time weather or historical climate data.

Surface algal blooms in the North Basin, 30 September 2004. This is what the regions of brightest green on the satellite images look like when you pass through them on the lake.

Wind and waves on the north shore at George’s Island on the 1^st of October, with the Namao tucked safely in the harbour, and a view of the storm from the lee passageway on the Namao.

12:50 25 September 2004 12:05 24 September 13:50 24 September 12:50 25 Sept. 2004

I posted the group of North Basin images on the right to show how quickly the apparent intensity of an algal bloom viewed from space can change. The first of the set were recorded at 12:05 and 13:50 on the 24^th. You can see the same patterns in the colours, but the green in the northeast quadrant, north of Georges Island, is much brighter in the earlier image. And in exactly the same region, there is no sign of the bloom a day later. The blue-greens actively regulate their buoyancy and work their way up into the bright near-surface environment when they can. I suspect that it was calm enough on the 24^th to do just that, though the winds may have started up sometime before the second image. (I don’t have the weather data for the 24^th, though I do for the 25^th. Environment Canada allows a few days’ delay between posting the latest 24 h weather and posting the historic climate record – the data for the 24^th will be posted sometime next week. You can get both current weather and historic climate by following the links at http://www.weatheroffice.ec.gc.ca/canada_e.html.) On the afternoon of the 25^th, winds at Norway House (just to the north of Lake Winnipeg) were strong westerly, gusting to 30 km/h all afternoon. With winds like that, there would have been 1 or even 2 m waves in the eastern region, so that algae that had floated to the surface would have been mixed back deep into the water column. The lesson I take from this is that these images are showing us only surface blooms. Any single image cannot indicate the absence of algae in the water column (it didn’t all die between the 24^th and 25^th – it was merely mixed downwards for the time being) although a bright green patch does definitely indicate its presence in abundance. Its only through a cumulative look at a lot of images each year – and knowing weather conditions associated with each -- that we can fairly compare frequency of blooms from year to year. But there are a lot of images, and these blooms can be identified on weather satellites that have been recording scenes of the lake every day or so since the early 1980s. So we’re doing just that – looking at as many cloud-free images as we can find each year and finding out what the historic record of satellite imagery can tell us about whether/how much the frequency and extent of such surface blooms may have changed over the last couple of decades.

By the way, for anyone who wonders. Yes, there are often two images of the lake on a given day. And then often not another for a couple of days, even without the problem of clouds. NASA now operates two satellites with the MODIS instrument on board. Each covers most of the earth each day. Unfortunately, Lake Winnipeg is centred in only about every third day’s image by each satellite – and that on the same day. When Lake Winnipeg is way off to the side, the image is so distorted as to be unuseable. You can see some distortion (they are only partly corrected) in the images on the 24^th compared to the 25^th, above. The satellite passed nearly overhead on the 25^th, but it captured the scene at 12:05 on the 24^th in its peripheral vision from way out over Saskatchewan.

13:10 21 September 2004 13:05 14 September 2004

No sign of the surface blooms of August in these September images, but it would have been difficult for algae to have concentrated near the surface given the very strong south winds of the last few days. The soft greens in the eastern and central North Basin do probably signal that there is still a lot of algae in the water. Once again, we’ll soon have first hand data – tonight the Namao is harboured at MacBeth Point, at the south end of the North Basin, and tomorrow its crew will head north, beginning a couple of weeks of sampling throughout the basin. Several of us will be joining them for the return cruise on Monday, and we should be back in Gimli near the end of the first week in October.

For those of you who read Helen Falding’s article in the Free Press a couple of weeks ago, you can see Limestone Lake clearly in this image. It’s the bright blue lake just northwest of Lake Winnipeg. In the article, Derek Ford of McMaster University explained the colour as the result of calcite precipitating – a phenomenon called a ‘whiting’ which he said turns the lake a beautiful chalky blue, or turquoise, or even emerald green depending on the temperature. It’s a karst lake, meaning that its in a terrain formed partly by dissolution of the limestone bedrock – hence the high calcium carbonate content of the water. Its also near a potential mine development, and the lake that the Canadian Parks and Wilderness Society are lobbying to have it incorporated in the National Park being developed along the shore of Lake Winnipeg. There is another pale blue lake to the southeast of Limestone, Clearwater Lake. (You can see the area a little more clearly in two other images wo images, from August of last year and June of this year). Its not as bright a gem as Limestone, but still a beautiful robin's egg blue from space. And from its colour at various times, I think that whitings occasionally colour Talbot Lake, the larger lake to the east -- and probably also parts of the Moose Lakes, although I suppose their colours to be a more complicated mix due as well to algae in the summer and occasional sediment resuspension events. I have wondered at various times if the pale greens that I often see in Lake Winnipegosis might be some combination of calcite and algae. Though they could simply be different algal communities than we regularly see in Lake Winnipeg.

12:25 31 August 2004

The picture has changed since mid-August (below). Where in early August the east side was all browns indicating either turbid water or water rich in dissolved organic carbon from the Shield watersheds (or very probably a mixture of the two), on the 31^st of August along the same shore there was a surface bloom of algae as much as 10 km wide from the Narrows to somewhere near Poplar River, a distance of something like 150 km. You can see the antecedents of this bloom from the Narrows up to Berens Island on the 13^th of August, but from Berens north at least to Georges Island there is no sign of it in the earlier image. Unfortunately, clouds prevent us seeing the fate of the large blooms in the northwestern part of the basin – as they have prevented us from seeing much of the lake through the whole last half of August. Again, I’ve included a vegetation index map on the left – with darkest green denoting most dense and most vigorous vegetation. Much of the bloom in this rendition is indistinguishably green from the forest just to the east of it. Its an indication that a thick green matte of algae completely covered the water surface there, on that day.

13:05 13 August 2004

An image recorded Friday the 13^th, when the Namao was probably near Georges Island doing fish trawls. The map on the right shows a vegetation index prepared by mapping the ratio of infrared light to red light. Chlorophyll in healthy green vegetation reflects infrared strongly, but absorbs red for photosynthesis. The vegetation index map shows increasing chlorophyll response in the colours brown (low) through yellow through increasingly dark green. The patches of these colours in Lake Winnipeg are surface blooms of algae, and in some parts, they are almost as green as some of the terrestrial vegetation around the lake – a sign that in those regions the algae is must be nearly carpeting the surface. Since water absorbs infrared light, for the vegetation index to produce even the browns and yellows out in the lake, it must be fairly dense on or near the surface. I expect that the Namao passed through one or both of the two blooms last week. We’ll know soon – they’re due back in Gimli tuesday.

11:20 5 August 2004

The bright yellow-greens signalling algal blooms are if anything a little more widespread in today’s image than a couple of days ago, at least in the region between Reindeer Island and Long Point. Mike Stainton (by satellite telephone this evening) tells me that the Namao sampled from Berens River to George’s Island today – along the track marked in red. (They set out yesterday, but had to return to Berens to get one of the science crew to the nursing station. Happily, he is well again, today.) Today’s cruise will have taken them through mostly the brownish water along the west side of the lake. In fact, near Berens Island today, they did not encounter the dense algal blooms that we have seen there several times in recent years, though they did pass through a small bloom day before yesterday. You can see it on this image – small swirls of green just a few kilometres north of the Narrows. Tomorrow, their intention is to sample two transects across the centre of the North Basin – the blue circuit. That’s “weather permitting”. Mike tells me that there’s a strong wind from the south rolling waves into the harbour mouth at George’s tonight. Anyway, when they do set out, their planned path will carry them across a huge algal bloom that look as dense as any we saw last year.

13:15 3 August 2004

If they are on schedule the crew of the Namao should have left Matheson Island this morning to arrive in Berens River this evening. That would take them through the tan-coloured region along the route marked in red above. That’s an area of fairly turbid water – turbid partly because it’s South Basin water carried north through the channel and partly because the region is shallow, at least compared to the basin north of Berens Island (generally only 10-12 m deep compared to mostly 16-18 m deep in the open North Basin) so that bottom sediments may be brought into suspension whenever there are strong north winds. It will be interesting to see the water chemistry data from the mouth of the Berens River. The dark browns along the east shore have been much more intense this year compared to last – that fits with the relatively high runoff that I think has been coming off the shield. Like many of the Shield rivers tributary to Lake Winnipeg, Berens River water is rich in dissolved organic matter – it looks like tea when you lift a glass of it from the river – and this year there appears to be a lot of it pouring into Lake Winnipeg. That’s interesting, too, from the point of view of algae. Mike Stainton has been telling me that high organic matter in the water can inhibit algal productivity. And further north: there is a remarkably sharply delineated bloom north of Long Point in this image. You could see it forming up in the 26 July image, below but its been relatively calm in the north today (you can check Lake Winnipeg weather buoy data at http://weatheroffice.ec.gc.ca/marine/region_06_e.html ) and the algae may have risen to the surface, making them much more clearly visible from the satellite.

11:35 25 July 2004 13:20 26 July 2004

Two images recorded while we were sampling on the first few days of the summer cruise. There’s a lot of variation in the colour of the South Basin. In the darker coloured regions – e.g the dark region northeast of Gimli – the water is clearer. The light browns are light reflected from the silt and clay particles, and thus indicate more turbid areas. Because algae need light to photosynthesize – that is, to convert carbon dioxide and water into carbohydrates for growth and reproduction – we would expect them to be more productive in the clearer compared to the muddier water, and indeed Mike Stainton’s continuous-flow oxygen and carbon dioxide data showed exactly that. In the darker region north-east of Gimli, carbon dioxide concentration on the 25^th was low compared to the browner, more turbid areas, and oxygen was high. Carbon from the carbon dioxide was being consumed by the algae, and the oxygen released back into the water. There were algae in the browner water around, too, but because of the higher turbidity they had less light to work with – so they were growing and reproducing more slowly – that is they were less productive. Mike’s carbon dioxide and oxygen measurements are a good measure of productivity wherever the Namao has passed, and the satellite images give us a good sense of how widespread were the conditions for high productivity on that day.

A press conference on board the Namao. On the left: Manitoba Minister of Water Stewardship, Steve Ashton, announcing $140000 support that will help pay the costs of keeping the Namao operating on Lake Winnipeg through this summer. Equally welcome was his announcement of additional resources for the riparian tax credit program, which supports the elimination of tillage and the limitation of grazing by livestock on lands adjacent to rivers and streams. The riparian tax credit now applies to land by lakes as well as streams. On the right: Claire and Keith identifying fish from a surface trawl – one of the many things they’ll be doing about 60 more times over the next month, only without the audience.

Pictures from the first days of the summer cruise. The first three show some of the sampling that we do at stations. 1. Alex taking a water sample at one of our stations, for determination of the plankton concentration in the surface water of Lake Winnipeg. 2. Claire about to lower a collection of profiling equipment – to log temperature, conductivity, turbidity, algal fluorescence and oxygen from top to bottom of the water column. 3. Pulling in a plankton haul – the material in the bottom of the net is the floating community of small plants and animals that were collected over a two kilometre haul coming in to the station. 4. Sampling from the continuous flow system. In addition to sampling at many stations, we make various water quality measurements continuously in transit between them, from an intake just under the water surface and in front of the ship. Turbidity and algal fluorescence data can be matched up with spectral reflectance observations, recorded over the undisturbed water foreward of the bow wave. The assembly visible near Stephen’s head holds a fibre-optic cable that carries light reflected off the water back to a spectrometer on board ship. In this picture, Stephen is taking a sample that will be filtered for laboratory determinations of suspended solids, chlorophyll and organic material – all of which contribute to the spectra of light recorded by the spectrometer on board as well as satellites passing overhead. These pictures were taken on the 25^th and 26^th of July in the South Basin of Lake Winnipeg – the same days as the two satellite images above.

11:30 18 July 2004 13:35 23 July 2004

Its been a cloudy summer over Lake Winnipeg; hence so few clear images. I’ve put up this pair just as the Namao crew are organizing for the midsummer expedition. We will sample around the South Basin for the rest of the week and head north the first week in August. If you compare the North Basin in these images with those from last July last July, you’ll see that we shouldn’t expect to find the same intensity of algal blooms at least so far this summer as we did in the midsummer cruise last year. This year we had a much cooler spring and early summer (has there been a summer yet?) so that spring heating of the lake will have been delayed compared to last year. We’ll surely find that the water is cooler for the time of year. Whatever we find, we’ll learn a lot more from sampling through a different kind of summer than if we’d seen a repeat of last.

For those who can get out there, the ship will be docked for an Open House (Open Ship?) – everyone welcome to come, look and ask questions -- at Victoria Beach on Saturday the 31^st of July and at Gimli on Sunday the 1^st of August. Further afield (weather allowing) there are tentative Open Houses at Berens River on the 4^th, at Grand Rapids on the 11^th and at Matheson Island 16^th of August.

11:55 28 June 2004

The lakes are for the most part clear of opaque clouds; however, there is a soft haze over most of the scene – either thin haze or possibly smoke from the west. Reports back from the spring cruise of the Namao indicate that diatoms still dominate the phytoplankton community. Due to the cold spring, they may well persist longer and delay any blue-green bloom to later in the summer than last year. Clouds permitting, we’ll be able to watch. The northern half of the South Basin, the Narrows region and much of the east shore are much less turbid than at the same time last year (see, e.g. 3 July 2003) when at least the South Basin and Narrows were more homogeneously brown in these MODIS images. Turbid brown water at the south end persists as a result of the high May and June flows from the Red River. The darker regions this year are probably waters much diluted by the high runoff from the shield drainages to the east. Certainly, you can see a dilute plume of Winnipeg River water passing through more turbid water along either shore of Traverse Bay.

11:30 16 June 2004 The Namao is conducting fish trawls, along with routine plankton, benthos, water quality and optical sampling, today near Georges Island (circled in red on the thumbnail above). That’s Stephen, Claire and Kevin hauling in the trawl net, with Namao crew member George in the background, a few days ago. I was just talking with Christina by satphone from the ship this morning, who confirmed what you can see in the image above – it’s a beautiful, clear day, and for once near calm. (There’ve been a lot of wet, windy and rough days so far.) A day like today is what was needed to collect data on turbidity and chlorophyll concentration in order to improve our estimates of both from satellite images like these. The ship spent the last few days sampling at stations in the basin north of Long Point, and will be returning south tomorrow, overnighting at Berens River, Matheson Island and Gull Harbour, and finally back in Gimli sunday, all going well. By the way, the crew have completed installation of all three weather buoys on this trip. You can access real time air temperature, pressure and wind data from the buoys at http://weatheroffice.ec.gc.ca/marine/region_06_e.html.

In both the image above and the one on the 9^th below, the muddy brown colour at the south end and in the Netley marshes is exactly what it looks like – muddy brown water from the Red River, which has been flowing high since the heavy rains of a week and a bit ago. And the pattern of dark water along the west shore of the North Basin looks a lot the same as last year at this time (10 June 2003) – this year we should get some well-positioned samples in that region to better understand the water quality (or plankton) differences underlying those colour patterns.

13:10 9 June and 12:05 10 June 2004 (North Basin only)

North Basin 27 & 28 May, 1 June 2004

Most of the west side of the North Basin had opened up by the 27^th. Although the ice covered much the same area on the 27th and 28th, you can see that it was much more broken up into separated pans on the 28th. By today, the 1^st of June, there appears to be very little ice except north of Long Point, and a big sheet attached to the east shore of Reindeer Island. Not a very good quality image, but I thought the crew of the Namao might like to know that its pretty well ready for them up there. They’ll be leaving for the north end in a couple of days.

Though I am studying the geography of plankton on Lake Winnipeg, it’s not supposed to be my only focus just now. Should you be interested, you can link here to a site describing my PhD research on sedimentation by the Linthipe River in Lake Malawi.