low flow vs. high flow
low flow vs. high flow
Yep, its quite a contradiction. We, as aquarists, dont know really if dwarf seahorses like low flow in the wild.. they just seem to do best in low flow setups in aquariums. All seahorses seem to like low flow because they like to stare down their prey for a few seconds before they attack it - hard to do if its whizzing by your head.
The grasses however, definitely do like flow and lots of it.. the more flow I direct over the grass the taller it seems to get. That link has been published and I can see the effects of the powerhead in grass in my tank thats in a low spot and in a high spot.
The papers dont say anything about plant density in high vs. low flow and thats a question I wish I could answer. Ie: In high current do the plants grow taller and exist more spaced out.. or do they grow taller
and form thicker beds? On my collection page in the website there is a shot of shoal grass (not a great picture, you'll have to trust me) and you can see that the plants are kinda spaced out. That was in the extreme shallow area. In deeper water the shoal grass was denser. Is it because of flow or is it something like disturbance by humans or wave action breaking the bed apart or.... ??
Back to the flow: I suspect the plants like high flow because more nutrients travel to them and some papers speculate that the movement of the water breaks up gradients around the leaves of necessary molecules like NPK and CO2. As the tide moves in and wave action brings in water the overall effect of all the seagrass beds together is to dampen the flow rate. They also trap dirt and algaes and other things as the water carries it over and through the beds.
Here's my theory: Think about the profile of a single grass bed in a single band in cross section. You'll have sand on either side and the grass growing in the center forming a nice rounded bulge. As water moves into the shallows from the sea the water will move into this bulge of seagrass. If its a thick established bed then some water will move through the leaves of the grass.. but the majority of it will be redirected over the top of the bed. Kind of like running into a semi permeable wall. I dont know how much of it will flow over the top, its just a theory at the moment, say 50% of what should be going through the bed if the water is hitting a nice big bed of turtlegrass.
This effectively compresses more water into a smaller space (the area above the bed) and accelerates it - the well known venturi effect. This would create an area of higher flow above the bed and lower flow within the bed. I think this is a possible explanation for having different flow preferences between the plants and animals.
Also.. in my latest Scientific American there was an article (Klimley, Richert, Jorgensen) on pelagic fish movements in the Gulf of CA and how large multi-species assemblages of fish would just appear from 'nowhere' at the underwater sea mounts. The authors speculated that the sea mounts became oasis in a virtual undersea desert due to the venturi effect. Lots of water from the surrounding ocean was being pushed over the sea mounts in high flow rates resulting in lots of plankton. Zooplankton concentration were increased by the venturi effect and the zooplankton was retained by other tidal properties over the sea mounts. More low-level trophic members means more high-level trophic members like snapper, amberjack, dolphinfish and scalloped hammerheads.
Maybe in a grass bed more plankton reaches it because of the venturi effect and the plankton are retained by sinking into the bed itself or clinging to the leaves.. or.....
Anywho, that's my theory. I may look at this later and want to smack myself but, hey, I can always write a retraction and claim insanity.
I really liked that question Fred!
>Sarah
Heres a link, you could always access by clicking the little red house in my posts and going to the marine side.
