Marine Snow: Whole versus Fragmented

Marine Snow can be described as sinking particles of organic matter or macro-aggregates, which are aggregates greater than 5mm in size. The marine snow collects at the surface water, where there is an abundance of bacteria and organic matter creating aggregates. The aggregates increase in density and eventually sink to the sea bed where due to the lack of light in the deep sea the aggregates are the primary source of carbon as many organisms are unable to photosynthesise. As the particles sink to the seafloor it is exposed to the currents of the ocean, the feeding and swimming of passing organisms; they are preyed upon by phytoplankton, zooplankton, bacteria, protists. These processes produce not only a plume but smaller particles (daughter particles) which still have the same composition, the division or fragmentation of the particles creates a greater surface area for bacteria to colonise. Daughter particles have a reduced density, so they sink at a slower rate. This means that smaller particles are more likely to remain in the surface area and according to Goldthwait et al (2005) remineralization is likely to occur.

Goldthwait et al (2005) tested two consequences of first time fragmentation on macroaggregates. 

•  The immediate release of dissolved organic carbon (DOC) and interstitial nutrients into surrounding seawater,

• The elevated solubilisation and remineralization of daughter-particle carbon due to the increased available surface area for bacterial colonization.

Approximately 200-300 aggregates of marine snow were collected by scuba divers at surface waters of Santa Barbara Channel in California during the summer months of 2002 -2003. Two types of experiments were carried out; 1 fragmentation experiment and 2 aggregate remineralization experiments. Each experiment was tested on the two treatments of aggregates (whole and fragmented) as well as a seawater control.

The seawater control was used to simulate the 10 aggregates which were added to 11 glass stopper BOD bottles filled with 300ml of unfiltered seawater. 4 replicates held the whole aggregates, 4 replicates held the fragmented and 3 held the control.

The fragmentation experiment analysed filtered particles for POC (particulate organic carbon) and PON (particulate organic nitrogen). Then Nutrient samples were collected from the filtrate, and analysed for their concentration of phosphate, nitrate/nitrite and ammonia.

3 replicates of 40ml DOC samples from the filtrate were stored at -20 then analysed using high temperature combustion, to determine the amount of the carbon dioxide present.

The Aggregate remineralization experiments consisted of 2 experiments, which were performed on 11 bottles kept at surface temperature and rotated end to end to keep aggregates in suspension

        Experiment 1: the bottles were re-suspended in unfiltered seawater, incubated for 3 days

        Experiment 2: the bottles were re-suspended in 0.2 µm filtered seawater and incubated for 5 days.

The amounts of solubilisation and remineralisation in whole and fragmented aggregates were compared, by assessing the pools of carbon from each. POC and DOC were measured and as a result TOC (total organic carbon) was determined (POC+DOC=TOC).

The authors concluded that fragmentation results in the immediate release of interstitial DOC and macronutrients to surrounding seawater. Solubilision rates were the same for whole and fragmented aggregates suggesting the surface of aggregates do not regulate bacterial colonization. Fragmentation causes a decrease in the changes of aggregate-associated carbon that come from the DOC release and slower sinking rate of daughter particles. Fragmentation also does not seem to accelerate POC degradation.

This paper originally interested me because it explained more about the presence and importance of marine snow in deep oceans. The fact that an aggregate of marine snow can divide into two smaller daughter particles and keep the same composition is incredible. Aggregate fragmentation is one of the primary removal mechanisms for sinking particles (Goldthwait et al 2005), this paper looks at its biochemical outcome and impact on the carbon cycle and this information and results could be useful in situations of large scale pollution and attempts at removing pollution.

Goldthwait S. A, Carlson C. A, Henderson G. K, Alldredge A. L,(2005) Effects of physical fragmentation on remineralization of marine snow, Marine Ecology Progress Series, Vol. 305: 59–65, Published December 23

http://www.int-res.com/articles/meps2005/305/m305p059.pdf