A few reasons why the Loch Ness Monster is not a plesiosaur:

Loch Ness can't support a population of large carnivorous animals

Loch Ness is a cold, deep lake, and has steeply sloping shores. This means that most of the water is cold and dark, and does not support much biological activity. If the Loch Ness monster is a plesiosaur, plesiosaurs would have to have survived for at least 65 million years. This could only happen if there was a substantial number of animals which would have formed a population large enough to avoid the problems of inbreeding. Loch Ness is neither big enough or productive enough to support such a population.

We would see them come up for air:

Although plesiosaurs lived in the water, they were air breathing reptiles. Even if they could submerge for a long time - marine turtles may be able to remain submerged for as long as five hours - they would still have to surface several times a day.

We would find the bones:

If there is a breeding colony of plesiosaurs in Loch Ness, some will die. Usually when a large animal dies, its carcase sinks to the bottom of the lake or sea. Bacteria in the guts of the animal generate gas, and after a while it floats back to the surface. After a while, the gasses escape and the carcase sinks to the bottom again, usually rather disintegrated. If there were plesiosaurs in Loch Ness, the floating carcases would occasionally be seen, and would sometimes be washed up on the shore. On the other hand, it is claimed that the waters of the loch are so cold that this fermentation is delayed until the carcase has disintegrated. If this is the case, the loch is probably too cold for a colony of plesiosaurs. Carcases decaying on the shore would leave bones behind, and if plesiosaurs have been living in Loch Ness for a long time, there should be plenty of bones to be found.

Loch Ness is too cold:

Reptiles do not generate their internal body heat, which is why they live in warmer climates. Marine reptiles in particular need to live in relatively warm water as they can't hibernate to survive cold winters. Large leatherback turtles are occasionally found further north than Scotland. A account (in Norwegian) of a leatherback found off the coast of northern Norway can be found here (thanks to Torfinn Ørmen for the information). However, this is presumably because they have been carried there by the Gulf Stream, which on the eastern side of the Atlantic is a warm, northerly current.

The eye-witness accounts of the Loch Ness monster are nothing like a plesiosaur:

There are many different descriptions of the Loch Ness monster, and not all of them agree with each other. Typical descriptions talk of a series of undulating humps. This would imply that the observers are looking at a mammal - reptiles can only move their bodies from side to side. The bodies of plesiosaurs were rigid, and could certainly not undulate either up and down or from side to side. Some descriptions refer to a neck held vertically out of the water. The bones of the neck, especially at its base connect in such a way that a plesiosaur could not hold its neck straight up. Other accounts talk of a animal walking on the shore of the loch. Marine turtles come onto land only to lay eggs, and find it very hard work. Plesiosaurs were less suited than turtles to move about on land.

They couldn't have survived in Loch Ness:

Loch Ness is only 10,000 years old. Plesiosaurs became extinct 65 million years ago.

My own view is that reports of the Loch Ness monster are very good for the Scottish tourist industry, but not backed up by any real evidence. One thing is for sure: even if there is a large animal in Loch Ness, it's not a plesiosaur.

An interesting titbit

(thanks again to Torfinn Ørmen): There is this little detail that not so many know about, I think. Loch Ness has in fact NOT a long sea serpent tradition. The tradition starts in the '30s with some sightings and some photographs looking like a basking shark and a pilot whale. The story with the misionary saint and the sea monster didn't happen in Loch Ness but probably where the river Ness runs into the Ocean. Loch Ness is about the only large lake in all the British Isles without a long sea serpent tradition. But after what you read in various media you would never believe it. It have been suggested that if there were something living and large swimming around in Loch Ness it could be a sturgeon. It has low metabolism and eats mud, more or less...

Steve Leonard's Search For...
...The Loch Ness Monster

First shown in the UK on BBC1, Sun 27 Jul 2003

"Steve Leonard leaves no stone unturned in his hunt for the elusive Nessie. He even goes as far as draining the Loch and abseiling to the bottom as he searches for caves which the prehistoric creature could hide in. He travels to the wetlands of Northern Australia to test the awesome bite of the monster's closest living relative - the Australian salt-water crocodile.
And he also gets up close and personal with the legendary Nessie. Using the latest computer graphics Steve throws on his wet suit and comes face to face with the monster. His conclusion, which involves a hidden camera style experiment which a coachload of unsuspecting tourists, is startling."

Not so much a criticism, more a bit of padding out: it seems to me that some details were missed out to 'improve' the story.

The programme referred to the paddles of pleiosaurs as being broad and short, adapted for fast manoevering. This is only part of the story. Some plesiosaurs, such as Cryptocleidus had short, broad paddles, others, such as Muraenosaurus longer and more slender paddles better adapted for fast swimming. (Robin O'Keefe published a short paper in 2002 exploring the differences in paddle morphology between plesiosaur taxa)

The idea that plesiosaurs lived in very cold waters seems improbable, especially as the fauna described by Ben Kear contains a large number of juveniles. Australia was a polar continent in the Mesozoic, but was too warm to support a permanent ice-cap. The climate as 'strongly seasonal'. Perhaps the plesiosaurs came south to breed in the warm summer months? It should be noted that although leatherback turtles are occasionally found far north, they can survive there because of the warm current of the Gulf Stream (see my comments above).

On a more critical note, crocodiles are not the closest living relatives of plesiosaurs. As archosaurs, they are more closely related to birds than they are to plesisoaurs. Although the affinities of plesiosaurs are rather uncertain, their closest living relatives are lizards and turtles. Crocodiles share certain aspects of their life-style with plesisaurs - i.e. they are aquatic carnivores - but in other respects, in particular the degree of adaptation to an aquatic life-style they are not so simliar.

The programme confused plesiosaurs and pliosaurs - understandable, as the popular terms and the scientfic terms carry different meanings. Popularly, plesiosaurs are the long-necked, small-headed forms and pliosaurs are the large-headed forms of the plesiosauria. So when we refer to plesiosaurs, we could either be refering to the whole family, incuding the pliosaurs, or just the long-necked plesiosaurs.

The pliosaurs were large and powerful marine predators. Although the geometry of the biting muscles is different from that of crocodiles there is little question that they could bite with awesome force. However, the plesiosaurs (i.e. the long-necked forms) had a relatively much less powerful, but faster biting action, and were adapted for feeding on smaller prey such as fish and invertebrates. Their teeth were long and slender, unlike the robust teeth of the pliosaurs.

I found the section on how we perceive the world around us, and how expectation can affect what we see of particular interest. I felt that the subject of scale - the size of perceived objects - could have been explored more, but perhaps that was edited out of the final version

At the end of the day, I'm not going to argue with Steve Leonard's conclusion!

A small population of small monsters?

An email from Sofía Fernández, a marine biologist from Chile raises the possibility that the loch could support a small population of small monsters. I'll try to get hold of the paper she mentions.

"You should read a paper about this subject, although I think it is rather difficult for you to find it. It's called "The population density of monsters in Loch Ness" (R.W. Sheldon & S.R. Kerr, Limniology and Oceanography Vol 17 number 5, september 1972). Basically it is a production study of the lake, and according to the calculations of primary (algae) and secondary (fish) production oobtained by the authors, Loch Ness is able to support a population of 10-20 monsters of aproximately 1,5 tons. The smaller the monsters, the denser the population can be. If the population of migrating fish is taken on account, the posible standing stock of monsters could be even higher. It is very interesting, and, it is scientific too. It may be a little bit complex, but the conclusions are clear. Loch ness could support a small population of small monsters that could be genetically viable. well that was it, hope I was some help."