Stirred not shaken: How turbulence affects marine food webs
Movement of seawater affects how marine bacteria absorb organic material.
New research shows that ocean turbulence directly affects the ability of microscopic marine organisms to recycle organic material back into the food web. Scientists John Taylor of Cambridge University and Roman Stocker at MIT found that there's a relationship between the natural movement of water in the ocean and the ability of marine bacteria to act as recyclers.
"The research provides a unique insight into how small organisms, such as bacteria, interact with their environment," says biological oceanographer David Garrison of the National Science Foundation's Division of Ocean Sciences, which funded the research. "The results will lead to a better understanding of microbial dynamics and nutrient cycling in nature."
The movement of seawater--including small whirls and eddies--affects how marine bacteria absorb organic material, such as that produced by phytoplankton. In the process of generating oxygen, phytoplankton produce waste matter in the form of organic material, some of which becomes dissolved in seawater. This dissolved organic material is then absorbed by marine bacteria foraging for food. Bacteria are then consumed by larger organisms. The organic waste material excreted by phytoplankton becomes part of the microbial loop - and contributes to the functioning of the marine food web.
Taylor's and Stocker's research revealed that a delicate balance exists between the extent of water turbulence and the recycling activity of bacteria--with high and low levels of turbulence linked to lower recycling rates. Their study also looked at how the physical environment of the ocean might help to select the most successful types of marine bacteria, which compete for nutrients.
Some marine bacteria have evolved the ability to swim. Swimming bacteria, known as motile bacteria, have advantages over non-swimming species in foraging for food. However, swimming is energy-intensive so motile bacteria swim only when it is worth the effort in terms of pay-back. Some bacteria can also detect and respond to chemicals dissolved in the surrounding water, an ability known as chemotaxis. The researchers compared the activity of motile and non-motile bacteria across environments that differed in levels of water movement and nutrient availability.
All water moves, but the extent of its turbulence varies widely in the ocean. Wind and waves strongly churn the water near the surface, while the level of turbulence is much lower in the deep ocean, says Stocker. The results show that the organic matter excreted by phytoplankton is patchy in the ocean, and is not as uniform as had been thought. Organic matter enters water in bursts, after which a natural mixing and stirring action comes into play and the organic matter is dispersed. The patterns of dispersal depend on the strength of turbulence and the extent of stirring and mixing.
"When you pour cream into coffee and stir it carefully, you get swirls of cream," says Taylor. "Something similar happens when bursts of organic matter enter gently moving water. The swirls of organic matter are easily accessed by swimming bacteria which surround and absorb it. "If you mix cream into your coffee vigorously, it quickly becomes evenly dispersed. When this happens in the ocean, the swimming bacteria have less to gain in seeking out the thinly-dispersed organic matter."
Bacteria get energy from organic matter--but they also expend energy in swimming. "Our research suggests that the optimum environment is one of intermediate turbulence intensity--when bursts of organic matter are stirred into thin filaments that can be exploited by large numbers of bacteria," says Taylor. "The best environment for motile bacteria is one in which the organic matter is lightly stirred into the water rather than vigorously mixed."
Does water flow effect directly impact nutrient levels in our reef aquariums? Perhaps there is an ideal flow that would help detrivores catch their food and help minimize nutrient levels? I don't think this is something we have really considered in the past with regard to setting up our aquariums?
This is cool info ! Thanks for the link. Is this a case of less is more ?
I think what it is saying that any movement is important but random is better. It's interesting. but I don't know why I'm surprised, but there are bacteria living in the water column. I think conventual wisdom was you have to have live rock or some form of filter material but, flow is important in this process. Random, eddie like flow. Anyone else get anything else out it.
I think Chuck is right to the extent that less flow is going to keep the nutrients in a more concentrated form for a longer period of time and give the bacteria a chance to feast. Strong random flow disperses nutrients and makes them harder for the bacteria to consume efficiently.
I'm not sure of the practicality of worrying about it from an aquarium design standpoint. The best way I can think to put it into practice is to avoid the common practice of aiming a powerhead at the places where detritus settles (presumably an area of high nutrient concentration). However, my gut is telling me it is better to get it out via mechanical filtration just like we have always been trying to do.
Actually After reading the papers and watching the video which I think applies to us more then anything that consitant current and not blasting current in the tank is very important which we already knew and am sure everyone would agree. I also agree with Jim that removing it from the DT is a must. My brain then went to my fuge which is where in my system the real exportation of nutrients occurs. In my new tank I added power heads and elevated my rock and skimmers. I am reading this as maybe I was thinking correctly but not completely . I added the power heads to keep detritus from settling on the bottom to keep it suspended and allow it to reach the skimmers better. If I am reading this correctly,adding the power heads is also allowing the bacteria a better chance to do it's job before the skimmers get busy doing the rest of the exportation. It also reaffirms Anthony Calfo's statement of fuge water volume for positive biological reaction . Calfo states in several of his books that if your fuge exceeds 1000 gph you are defeating the intention of the biological process. I found the papers to be very interesting and help me to Decied to keep the power heads in the fuge and skimmer chamber that I was debating about removing them. It also makes me think not using a DSB was a good idea. That's my take and my .02.