The Gulf of Mexico Dead Zone

Primary production can be controlled by several factors.  Sunlight, availability of nutrients and the stability of the water column are the most important.  Areas that experience a change in any of these factors can experience a change in the amount of available phytoplankton.  Whenever there is an abundance of nutrients such as in areas of upwelling, the phytoplankton population can explode.  This is called a ‘bloom’.

While phytoplankton blooms can be beneficial to an ecosystem, if the bloom is sustained over a long period of time, there can be negative consequences.  The increased phytoplankton population will eventually die and sink to the bottom of the water column.  Bacteria will decompose this biomass and, in the process, consume most of the available oxygen.  This results in areas of low oxygen (called hypoxia).  The hypoxic areas result in zones where there benthic communities are lessened or completely missing.  The popular press calls these areas ‘Dead Zones.’

In the Gulf of Mexico at the mouth of the Mississippi River is one of the largest dead zones in the world.  This results from a spike in Nitrates and Phosphates in the river water every spring.  These nutrients are the result of agricultural runoff from the Mississippi watershed.  Increased nutrients results in a massive phytoplankton bloom and an area of hypoxic conditions.

Retrieved from

This zone has a significant effect on the indigenous shrimp population of the Gulf.  Commercial fisheries are affected as well as other benthic species.  The only solution proposed is to seriously curtail the use of Nitrate/Phosphate fertilizers in the American Midwest.  Even then, it will take years for the Gulf of Mexico Dead Zone to shrink.  As it is, it is only getting worse every year.  Until policy change is enacted, there will continue to be an area of the gulf the size of the state of New Jersey that will continue to be barren of anything larger than algae or bacteria.


Bruckner , M. (2011, March 12). Carleton college. Retrieved from

Rabalais, N., Turner, E., & Wiseman Jr., W. (2002). Gulf of mexico hypoxia, a.k.a. “the dead zone”. Annual Review of Ecology and Systematics33, 235-263.





Facts of Ocean Life

The marine environment is very different than the terrestrial environment.  That might be surprising to the less sharp among you, but I’m sure most of you were already well aware of this fact.  These differences lead to a huge range of different organisms, (called biodiversity). The biodiversity of the aquatic environment dwarfs that found on land. This fact is accounted for when the wide range of conditions found in marine environments are considered. One of the most important factors to consider is light. How organisms interact with light plays a major role in where an organism lives and what it’s made of.


On land, there are few environments that aren’t affected by light. This is not so in the ocean. The ocean can be divided into two distinct zones. The photic zone, where light is plentiful and photosynthesis is the major biochemical cycle and the aphotic zone, where chemosynthesis is the major biochemical cycle.

Between the aphotic and photic zones is an area of low light called the twilight zone.  Below this zone, there isn’t enough light for photosynthesis to occur. That doesn’t mean there isn’t light. Light becomes a major resource in the deep ocean, it just isn’t sunlight.  Bioluminescence is very common in the ocean depths.  Organisms generate light to attract mates, hunt and confuse predators.  Bioluminescence is produced when the enzyme luciferase acts on luciferin. It is believed that most organisms in the deep ocean use bioluminescence. Even bacteria use bioluminescence to attract hungry fish. When the fish eat the bacteria, they give them a safe, nutrient rich environment to live (their gut).

There are other ways for an organism to use light besides generating it. Often organisms in the ocean will take advantage of color to hide from or confuse predators. The worlds greatest camouflage artists reside in the ocean. Flounders, octopuses and cuttlefish are able to use structures called chromatophores to change their colors to match their surroundings.

You are awesome for hovering over this picture.  Give yourself a hug!

Here a flounder demonstrates an astonishing ability to match its surroundings

Another way that organisms use coloring to hide in the open ocean is countershading. Countershading is when an organism is darker on the dorsal region and lighter on the ventral side.  This makes it difficult to pick it out from above because it blends with the darker ocean and difficult to see from below because the lighter belly doesn’t stand out with the lighter ocean surface.

Be careful the shark doesn't bite you!  Rawr!

Here a shark demonstrates countershading

The final way an organism can use color to hide is to completely eliminate color. By making themselves mostly transparent, an organism can hide in plain sight.

Introducing, the king of Hide 'n Go Seek, the transparent squid!

The Transparent Squid, where'd he go?

The Carbonate/Bicarbonate Buffer System.

Carbon dioxide plays a vital role in the chemistry of sea water.  When atmospheric carbon dioxide is dissolved in seawater, carbonic acid (H2CO3) is formed.  Carbonic acid is diprotic, which means in has two H+ ions to donate to solution.

When the first proton is donated, HCO3 otherwise known as bicarbonate, is formed.  Most of the carbon (around 88%) of the carbon in the ocean is in this state.

If bicarbonate donates its second proton (H+), it becomes a carbonate (CO32-) ion.  About 11% of the carbon in the ocean is carbonate.  The other 1% is dissolved Carbon Dioxide.

The balanced equation is below:

 As you probably noticed, the chemical reaction has multiple steps and can go in both directions.  The bicarbonate acts like a buffer in the ocean.  What this means is that this system is very resistant to changes in pH.  If you were to add an acid to the ocean, the excess H+ ions would simply drive this reaction to the left and produce more products on the left side of the reaction.  If you added a base, the deficiency in H+ ions would drive the reaction to the right.  The system is resistant to net changes in the pH of the system.

Free Calcium (Ca2+) ions can sequester the Carbonate ions to produce Calcium Carbonate (CaCO3)  This calcium carbonate is used to make shells and tests for many organisms in the ocean.  It also accumulates above the Carbonate Compensation Depth as Calcerous ooze.

The Carbonate/Bicarbonate buffer system is an important way for the ocean to maintain chemical equilibrium. What would happen if the amount of CO2 in the atmosphere were to sharply rise?

The increased atmospheric CO2 as a result of burning fossil fuels has driven this entire reaction to far to the right.  This means that there is an excess of H+ ions in the ocean and the pH of the ocean has been driven down.  This is called ocean acidification.  This is just one more negative effect of Global Climate Change.



Anatomy of the Beach

The beach includes all sand and sediment that accumulates throughout the seasons. The diagram below tells you the different regions of the beach (called the beach profile).

The shore can be broken in to 4 major regions.

Offshore is the region of the shore that is never dry.  It runs from below the low tide line (called the low-tide terrace) to the deeper ocean.

Nearshore/Foreshore is the region that runs from the lowest low tide line to the highest high tide line.  This area includes the majority of the wave action.

The Backshore is the area of the beach that stays mostly dry year-round.  The buildup of sand on the beach is called a berm.  As the sea level changes long-term, ancient beaches can be covered in rising sea water.  These places are called drowned beaches.

During the summer, waves are gentle and build up a berm close to shore.  During the more violent and active winter months, the summer berm is erased and a winter berm is formed further up the shore. The trough between the berms is called the berm scarp.

Due to the movement of the tides and longshore transport, a buildup of sand accumulates between the low and high tide lines.  This buildup makes for a shallow water region in foreshore called the low-tide terrace.

The action of the waves carves the shoreline and erodes the beach until cliffs form.  The size of the sediment and the vigor of wave action determines the size of the beach.  The larger the sediment, the harder it is to carry away and the larger the beach.  The finer the sediment grains, the easier it is to carry away  and the smaller the beach.

Offshore, the sediment can accumulate in bars due to longshore transport.  If the bar grows enough to pierce the surface of the water, it can become a barrier island or a sandbar.