Question about aquatic plants:

[XemaSab]

Why do vascular plants not typically grow in open water habitats?

I'm reading an environmental document, and Kipling would be proud of the rationalization for this.

Thanks.

[TechBear_Seattle]

1. The explanation I've usually heard is...

The vascular system of plants evolved in order to 1) provide support and 2) transport water, minerals and sugars between the roots and leaves. Aquatic plants have not faced these evolutionary pressures, so aquatic plants have had no need to evolve them. Those aquatic plants that are vascular generally have land-based ancestors.

[Buzz Clik]

2. The lack of oxygen in the root zone in water is a big problem.

Metabolism drives all plant functions, including the uptake of critical nutrients. In aquatic systems, oxygen levels are too low for typical vascular plants to function at a sustainable level. Aquatic plants have developed the mechanism to deliver oxygen to the roots by piping it down from the leaves.

[XemaSab]

3. This is the explanation I was given back in the day

This document is trying to sell some nonsense about light being the limiting factor. Hmm.

[Buzz Clik]

10. Is this document something you can forward?

[XemaSab]

11. Here's the problematic passage:

Sad pre-edit:

Open water habitat consists of deep-water areas that exhibit perennial inundation. Vegetation is limited to the edges of these features because the water depth inhibits sunlight from reaching the bottom where vegetation would typically be rooted.

Happy fun edit:

Open water habitat consists of deep-water areas that are usually underwater throughout the year. Vascular plant species are typically limited to the edges of this habitat because the water depth inhibits sunlight from reaching to the channel bottom where vegetation would typically be rooted.

(And yeah, the happy fun edit is still not perfect by any means.)

[mike_c]

4. vascular plants DO grow in open water....

Most rooted macrophytes need to maintain gas filled mesophyll for photosynthesis, so they either either have surface tissues in contact with the atmosphere or spongy aeroenchyma tissue. Non rooted macrophytes float, so it's not as much an issue for them. And of course, rooted plants are restricted tot he littoral zone, where sunlight can penetrate-- if by open water you mean pelagic zones, then light penetration is the limiting factor, but floating macrophytes can still grow there, and do.

Oh, I'm also assuming freshwater habitats. I don't know much about marine plants.

[kestrel91316]

5. I just KNEW you would chime in here, lol. The Science Guy.

[XemaSab]

6. "Open water" is given as a habitat type

for (cough) a certain river that flows through a certain county between us (cough) and the explanation for lack of veg in the open river channel is that light doesn't penetrate through the water column. You're right, light penetration is why pelagic habitats don't have veg. And yeah, I was thinking about stuff like hydrilla and I would suppose the limiting factor there would be swift water movement, as well as the dynamic flow regime (cough).

My understanding is that most marine plants are algae, which are nonvascular.

Man, you want to read a steaming pile of BS, read an environmental document sometime.

[mike_c]

8. yeah, I was thinking more about lentic habitats...

...but in your case, I'd think substrate turnover/dynamism is an important consideration. It's harder for macrophyte propagules to establish or persist in the central channel of lotic systems. Light limitation probably does play a role, but that would depend upon depth and sediment load. It's hard to envision any of our local rivers being severely light limited during late summer flows unless the sediment load is horrendous.

[XemaSab]

9. A few of the nearby rivers seem to carry a heavy sediment load

near their mouths.

I'm thinking of the Klamath, the Sacramento, and so forth, but the project area is not only up in the hills, it's behind... obstructions... in the river. Sediment load is probably not a big deal there.

Someone should do a class over there on reading and writing environmental documents, 'cause they're all shit.

[mike_c]

12. lol-- is it a lake or a river?

I mean, if it's behind an impoundment it's probably more lentic, in which case there might be a real pelagic zone that's light limited. If it's a relatively low-order river, I'd be rather skeptical. Substrate dynamics are completely different in each. In lentic waters light limitation is usually more important. In lotic systems bed movement and flow velocity are more important.

[XemaSab]

13. I made a mistake in the phrasing

It's below the impoundment.

[hunter]

7. Huh? They do grow in open water habitats. Very densely sometimes.

Hmmm, maybe that's the answer... if the water is entirely covered with vascular plants it's not open water, right?

If vascular plants don't grow in open water it's because the wind is blowing them away to rot somewhere, the currents are carrying them away to rot somewhere, it's cold, there is a limiting nutrient of some sort, or something is eating them.

Otherwise any open water very soon looks like this:

water hyacinth invasion, Lake Victoria, Kenya

http://en.wikipedia.org/wiki/Water_hyacinth

Invasions of exotic water hyacinth species have been spectacularly destructive worldwide, but other exotic water plants, rooted or not, can do similar damage.