Zooplankton Tow Sample Analysis

We conducted our second indoor lab assignment on October 11, 2010. We analyzed the two plankton tow samples (surface and oblique) from the October 4, 2010 outdoor lab assignment. Surface tows were characterized by being able to see the net at the surface. Oblique tows were characterized by the net being at least 4 m below the surface. The objective of this lab was to utilize dissecting microscopes to assess the abundance of copepods from two milliliter (ml) water samples from surface and oblique plankton tow samples.


Copepods are vital marine organisms that prey on phytoplankton in coastal ecosystems. They serve as important prey items for shrimp, small fish species, and crabs.
The plankton samples were collected using bongo plankton net with 1 mm (500 µm) mesh and collection container attached in the middle. The bongo plankton net was towed behind the boat at a speed of 0.77 m/s. After each tow, the bongo net was brought aboard and the collection container was removed from the net. We used saltwater to wash the plankton samples from the collection container into separate containers labeled either “surface” or “oblique”.

In order to assess the abundance/ number of copepods per sample, 1 ml pipettes were used to remove a 1 ml water sample from the surface and oblique plankton tow samples. One water drop of the plankton samples were placed in 36 miniature squares of a water sample tray. Each tray was observed under a dissecting microscope. The number of copepods per miniature square was calculated for both plankton tow samples.

Both the surface and oblique plankton tow samples produced low numbers of copepods and other zooplankton.

Estuarine Sediment Grain Size Determination

Marine geologists analyze sediment grain sizes to understand the composition of earth within a survey site. Through this analysis, they can predict sediment composition changes when the information is combined with other datasets (weather, land use). On September 20, 2010, the research methods class conducted an estuarine sediment grain analysis exercise.

Sediment cores collected from a previous survey (blog post 9-10-10) were procured for this exercise. A stacked series of cylindrical mesh sieves and water were used to manually separate the sediment grains according to the following sizes:

Coarse sand -1000 µm
Medium sand - 250 µm
Fine sand - 125 µm

The contents of each sieve were rinsed with water before they were transferred into pre-weighed collection dishes and dried in a laboratory oven for 7 days. The samples were weighed after drying.

The Station A core had a calculated dry weight of 8.41 g. Coarse sand was in 2.26 g (27%) of the sample. 3.06 g (36%) of the core was medium sand. 3.09 g (37%) of the core consisted of fine sand (Figure 1; Table 1).

The Station C core had a calculated dry weight of 62.13 g. Coarse sand constituted 14.57 g (23%) of the sample. 9.32 g (15%) of the core was medium sand. 38.24 g (62%) of the core consisted of fine sand (Figure 1; Table 1).

The Control core had a calculated dry weight of 82.36 g. Coarse sand was in 2.11 g (3%) of the sample. 10.66 g (13%) of the core was medium sand. 69.59 g (84%) of the core consisted of fine sand (Figure 1; Table 1).

Figure 1 - Estuarine sediment grain size to weight composition. 

Skidaway Island, GA

  

Table 1 - Dry sediment weight of sieved samples.  Skidaway Island, GA.

	Station A	Station C	Control
Coarse sand 1000 µm	2.26 g	14.57 g	2.11 g
Medium sand 250 µm	3.06 g	9.32 g	10.66 g
Fine sand 125 µm	3.09 g	38.24 g	69.59 g
Total weight	8.41 g	62.13 g	82.36 g

In conclusion, the sediment cores in this exercise contained high quantities of fine and medium grain sand.

A plausible theory: This area experiences high tide differentials (8 feet / 2.4 meters twice daily); yet the energy of the water as it enters and leaves the study site is low. As a result, sediment grains of smaller size are able to settle and accumulate since the water is flowing at a relatively steady rate.