DATA INTERPRETATION
NOTE: Please read
the STATIONS
section of the website before working on this section. CLICK HERE to return to
the opening page.
Below is a summary
of oceanographic conditions that are typical of our coastal
ocean during the following times:
o
Normal
Southern California Winter
o
Normal
Southern California Summer
o
El
Nino year
o
La
Nina year
o
Harmful
Algal Bloom (HAB)
o
Polluted
Ocean
Use these descriptions as a guide to
interpret data acquired during the Advanced Floating Lab boat trip.
Normal Southern California Winter
The productivity on the Southern
California coast is controlled by coastal winds and the strength of the thermocline. In the winter, there is usually a weak
thermocline (in other words, little temperature difference surface
water and deep water) and coastal winds are from the northwest. Both conditions are ideal for the upwelling
of nutrients that can be present on a healthy seabed. A healthy
seabed is generally brown or green in color, has a rich earthy/ocean
smell, and has a significant number of decomposers such as pink brittle
stars. The water sweeps across the
healthy seabed and picks up nutrients.
This nutrient-rich water then rises up to the euphotic
zone, where the phytoplankton use the
nutrients, along with sunlight, to photosynthesize and reproduce in vast
numbers. Nutrients also come to the ocean
from land-based runoff (rivers flowing into the ocean), although this is
secondary in importance to upwelling for the supply of nutrients. A Secchi Disk under such conditions typically shows
greenish water color and moderate to low water clarity due to the abundant
phytoplankton (particularly diatoms). Oxygen
levels may be quite high in surface waters due to phytoplankton
photosynthesis. Most fish
species caught are in their normal range.
A summary of our normal southern California
winter conditions:
o
High primary productivity (lots of photosynthesis
and reproduction by phytoplankton)
o
Weak thermocline (little temperature
difference surface water and deep water)
o
Strong upwelling
o
Secchi disk will be green with moderate to low
water clarity
o
Plankton: large amounts of diatoms,
although usually few dinoflagellates
o
Seabed: healthy (hopefully)
o
Seawater properties: oxygen levels normal to high in
surface waters; pH and salinity in normal range
o
Fish species: most fish species caught are in their
normal range
Normal Southern California Summer
The productivity on the Southern
California coast is controlled by coastal winds and the strength of the thermocline. In the summer, we often have a strong
thermocline (a large temperature difference between warm surface water
and cool deep water) and the coastal winds often come from the southwest. This creates conditions that are not
conducive to upwelling. The
surface water is warm and therefore less dense than the cooler water down below,
which makes it difficult for nutrients to rise up to the surface to be used by
phytoplankton. Therefore, phytoplankton
productivity is low, and this means that the Secchi Disk
typically shows blue water color and high clarity. There may be lower dissolved
oxygen than in winter because of lower rates of phytoplankton photosynthesis. Most fish species
caught are in their normal range.
A summary of our normal southern California
summer conditions:
o
Moderate to low primary productivity (less
photosynthesis and reproduction by phytoplankton)
o
Strong thermocline
o
Lack of upwelling
o
Secchi disk will be blue with high water clarity
o
Plankton: moderate to low amounts of phytoplankton
and zooplankton
o
Seabed: healthy (hopefully)
o
Seawater properties: lower oxygen than in winter
(but still in normal range), pH and salinity in normal range
o
Fish species: most fish species caught are in their
normal range
El Nino years
During El Nino
periods, a strong thermocline develops and persists most of the year. The warm surface water is too buoyant to
allow the cold, nutrient-rich deep water to rise up to the euphotic zone. Therefore, very little upwelling of nutrients
occurs, and productivity is thus very low.
Fish may migrate out of their range to find food, as food supplies are
generally low, and fish that normally live in warmer waters to the south (like
along the coast of Mexico), may migrate north to our area.
A summary of El Nino conditions:
o
Low primary productivity
o
Strong thermocline all year long, even in winter
o
Lack of upwelling
o
Secchi disk will be blue and the water very clear (at least when there is not sediment from rainstorms in the water)
o
Plankton: low amounts of phytoplankton and
zooplankton
o
Fish species: low numbers, and some are exotic (not
normally found in this area)
La Nina years
During La Nina
periods, surface waters are unusually cold and the thermocline is very
weak. Productivity may actually be
higher than in normal years. Since La
Nina years often follow El Nino years, fish populations may be recovering. La Nina conditions are similar to the
conditions of a normal southern California winter.
A summary of La Nina conditions:
o
High primary productivity (lots of photosynthesis
and reproduction by phytoplankton)
o
Weak thermocline (little temperature
difference surface water and deep water)
o
Strong upwelling
o
Secchi disk will be green with low to moderate water
clarity
o
Plankton: large amounts of diatoms, although
usually few dinoflagellates
o
Seabed: healthy (hopefully)
o
Seawater properties: oxygen levels normal to high in
surface waters; pH and salinity in normal range
o
Fish species: population variable, recovering from
previous El Nino
Red Tide / Harmful Algal Bloom (HAB)
The main indicator of a harmful algal
bloom (commonly known as a “red tide”) is the great abundance of dinoflagellates. There may be few
other plankton in the water even though we may have a weak thermocline and a
healthy seabed. The Secchi disk will
show a brown color and low water clarity.
o
Abundant dinoflagellates
o
Secchi disk: brown
color with low water clarity
o
Seabed: unaffected but healthy (hopefully)
o Fish species:
most fish species caught are in their normal range
Polluted Ocean
In ocean areas that are polluted or
contain stagnant water, there may be lower-than-normal dissolved oxygen levels.
The seabed may be black or dark in color and have a rotten egg smell. Even if upwelling is occurring, the necessary
nutrients may not be delivered to the euphotic zone, and productivity may
therefore be limited. In extreme cases,
the pH may not be in the normal range.
A summary of Polluted Ocean
conditions:
o
Seawater properties: low dissolved oxygen, also
possibly abnormal pH
o
Seabed: unhealthy, rotten egg smelling, black/dark
in color, few decomposers
o
Fish species: low overall numbers
Contents of this page modified by Keith Meldahl
based on materials developed by John Turbeville, Mira Costa College.
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