Open water season, 2003
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.
They are RGB colour composites of MODIS 250-m resolution data with Channel 1 (670 nm) displayed as red, and Channel 4 (565 nm) as green and Channel 3 (479 nm) as blue, i.e. approximating colour as people perceive it, except with the colour contrast a bit exagerrated. The brightest greens in the water column are indicative of algal blooms but a bit of "greenness" is not necessarily due to algae at all. That's because as the lake varies from clear thru low to moderate to high mineral sediment concentration, the change in relative spectral attenuation of light in the water column causes reflected colours to shift from dark blue-black (clear water, most of the incident light absorbed, only a bit of blue reflected) through darker blues to green to the greenish-browns and tan (turbid water, reflecting a wide mixture of colours, "brownish" to our eyes). With these particular renditions of MODIS images is that the intensity of the green is not quantitative, so its hard to be sure of the relative effect of low-moderate mineral sediment versus moderate algal density.
Scroll down, and click on underlined dates below to view a larger copies of the images.
11:55 10 June 2003 In viewing many AVHRR images spanning the last decade and a half, I don’t remember seeing the South Basin clearer (darker blue) than even the furthest offshore waters in the North Basin, as it is in this image. This condition persisted from soon after the lake was clear of ice until at least a few days after this image was recorded. You can trace relatively clear (darker) water – mixed with more turbid water (light tan) from the west-side bays -- up through the Narrows and along the east shore pretty much as far as Berens Island. The broad band of brown along the east shore north from Berens almost to the outlet is different again in tone – greyer-brown – and coincides with the some of the highest diatom concentrations that we observed in the June cruise of the Namao. Alternatively, the grey-brown colour may be due to mixing-in of brown, DOC-rich water from the eastern watershed. The greyish cast over parts of the western North Basin (e.g. south and southeast of Long Point) may be mist obscuring the lake surface. Nonetheless, there is visible a grey-brown patch in the centre of the basin north of Long Point – possibly also a region of more intense diatom production. Interestingly, it corresponds spatially to the centre of the turbid plume visible on AVHRR images recorded in late May (14:38 25 May 2003) soon after the North Basin was cleared of ice.
The 21st of July image appears to show widespread algal blooms forming in the North Basin, with particularly intense greens both between Reindeer Island and Long Point, as well as near the north shore (partly obscured by clouds). The soft browns beginning north of the Narrows and extending to the south end of the lake indicate relatively turbid waters (pale yellow-brown in Fisher Bay, west of the Narrows, is probably the most turbid region in this scene). And the very dark brown in the mouth of several east shore rivers (e.g. Berens River) is probably due to the deeply red-brown waters from the wetlands in their watersheds.
MODIS images of the North Basin recorded on the 18th, the 21st and 22nd, and the 27th and 28th of July. (Unfortunately, the 22 July image suffers badly from misregistration of pixels, causing duplicate bits of image, most obvious in the repeats of sections of north-south shoreline. The images are not geocorrected.) The bluer regions in the lake indicate relatively clear water. The most intense greens are due either to high concentrations of algae, or alternately, to algae floating at the surface, where it would be most effective in imparting colour to the image. It is interesting how persistent are the patterns they form, particularly the pattern formed by two large patches of green north of Long Point (A and B on the 2nd image from the right). There is already a hint of these future patterns in the 18 July image, and they are fully formed by the 21st. Through the 27th and 28th they are hardly changed. On the other hand, the patch of green south of Long Point does appear to have drifted eastward over the period represented by this set of images. Notice that on the 28th, the bloom at “C” is a few kilometres east of its position on the 27th. In a lake so open to the force of wind, this drifting seems to me to be expected; it is the week-long-plus persistence of these large patterns that surprises me.
11:55 28 July 2003 The folks on the Namao sampled near the mouth of the Red River on this day, confirming that chlorophyll concentrations are indeed higher in the greener waters there. In this image, the Winnipeg River water can be seen as a dark plume moving up along the east shore of Traverse Bay. The west side is much the greenest region in the South Basin – hopefully, we will confirm whether that indicates higher algal concentrations when we sample in that region on the 31st. Hedy has some new microphotographs of the phytoplankton in samples brought back from the Namao working in the South Basin and the Narrows.
11:55 8 August 2003 At about the time this image was recorded we on the Namao were cruising towards one of those clouds just south of Long Point in the North Basin. (Sorry, even a 30 m vessel is too small to see in a 250m-squared pixel.) And a couple of hours later we had to tarp the equipment and hide from an intense rain shower.
For those of you who might be interested in the colour spectra underlying these MODIS images, I have attached a page showing the spectra of light reflected from the water surface in front of the Namao on the same day that the image below was recorded.
11:40 15 August 2003 On this cruise, for the first time we have
observed severe oxygen depletion in the bottom waters of Lake Winnipeg. This is probably the most significant and
worrisome finding in the last decade of (sporadic) research and monitoring on
this lake. Although it has generally
been assumed that Lake Winnipeg is well-mixed, there was a thermocline a few
metres above the bottom in many of the 1994 profiles in the North Basin. On
this trip, we observed a 15oC water mass about 1-2 m thick below a
thermocline at 13-15 m depth, separating the cooler bottom waters from a
well-mixed upper water column at 21-22oC. Based on the bottom water’s temperature, it
had prob. not mixed with oxygen-rich waters since mid-June. Oxygen values were
~8 ppm in the upper water column, but only 2-4 ppm in the bottom waters.
Moreover, the carbon-dioxide concentration below the thermocline was elevated
about 5X the concentrations in the upper waters. The upper waters were phytoplankton-rich
-- chlorophyll concentrations in excess
of 20 ug/L were common -- and from looking at satellite images, was even richer
in mid-July, so that there was probably an abundance of organic material
settling into the isolated lower water mass.
If this thermocline persists through the rest of the summer, oxygen
could well be completely depleted in the North Basin bottom waters before lake
turns over in the fall. We intend to repeat the whole lake cruise in late
September, and will follow up then.
If complete anoxia develops, and especially if this becomes a common occurrence in future years, there is great danger to the fishery. In addition to the possibility of fish kills when this water mixes upwards, the bottom-dwelling insect population on which much of the fish population – in particular, the whitefish -- depends is in jeopardy. This finding puts a new level of urgency into the push to reverse growing eutrophication of Lake Winnipeg. Chlorophyll concentrations (our index of the amount of phytoplankton in the lake) are already higher in Lake Winnipeg than in the worst years in Lake Erie. Sedimentation and decay of these large masses of plankton, the inevitable result of increasing phosphorous loading to the lake, can only exacerbate oxygen depletion below such thermoclines. This finding also lends urgency to our need to understand physical and biological processes in the lake, processes that have the potential to transform eutrophication from an aesthetic problem on our beaches to a deeper ecological and economic problem.
11:35 24 August 2003 In the two images above, the intense yellow-green that coloured much of the North Basin in earlier images has receded, indicating that the intense algal blooms that occurred through July and early August have died back for now. The greenish-brown colour along the east shore of the North Basin is less like the intense greens that we saw in July and more like in spring – at a time when diatoms rather than green and blue-green algae dominated the phytoplankton community in the North Basin.
The plume along the north shore appears to be sediments from erosion of the steep clay banks that form much of that shore. At least, it has the colour pattern (light tan near shore grading to soft green offshore) that I have come to expect in these images where there is a gradation from higher to lower suspended sediment concentration in the lake. It is a result of the greater attenuation of reddish compared to bluish light in the water column. Red light is absorbed quickly by the water; blue penetrates relatively deeply. In clearer waters, a greater proportion of light is returned from deeper in the water column – so that the proportion of blue to red increases as the water grows more clear. Hence, where shore erosion is the source of the sediments, and where water grades from more turbid to clearer as it gets further from the source, the colour shifts from brown (or tan, in this case) through green, ultimately to dark blue. (For the colour of really clear water, look at Clear Lake in Riding Mountain National Park, near the lower left of this image.) And that is why one has to be careful in interpreting the greenness of water in these images. While the brightest greens in the July and August images were surely due to algae in the water column (we measured high concentrations on the August cruise) – some of the paler greens in these images can be due to silts and clays, even though they are not in themselves, at least as dry particles, at all green.
The brightest green plumes are even at this late date still associated with high algal concentrations. A week earlier, when the Namao cruised from Berens River up the east side and then across to Grand Rapids, north of Long Point, we were still recording chlorophyll-a concentrations from 5 to 30 ug/L, in planktonic communities dominated by diatoms. In 4 October we recorded similarly high chlorophyll-a, and in fact recorded it gradually concentrate at the surface over the course of several hours while we were repeat-fish-trawling an a single transect. Although the ranges of algal concentrations were similar in the two basins, blooms appear more dramatic in the North Basin where they are not obscured by the higher sediment load that occurs in the South.
11:25 29 October 2003 The first snows of coming winter (felt like winter was already here in Winnipeg on Hallowe’en) blanket the Interlake (the land between Lakes Manitoba/Winnipegosis and Lake Winnipeg) in this image. DOC-rich brown waters are particularly prominent at the mouths of the Bloodvein, Pigeon and Berens Rivers along the east shore north of the Narrows. (Some of the bog-lands drained by these rivers are nicely distinguished by their colour – the sphagnum of the treed muskegs showing autumn brown among the surrounding coniferous-covered, better drained uplands.) Notice how the Winnipeg River water below its mouth (in Traverse Bay at the south-east corner of the South Basin) is not so brown-black as the waters below the mouths of these smaller Shield rivers. It is by comparison a clear-water river; the smaller ones viewed from a small boat are a deep, very dark reddish brown.