Coastal Oceanography, Larval Behavior,
and the
Cross-shelf Transport of Larvae
Alan Shanks
University of Oregon
Oregon Institute of Marine Biology
Charleston, Or 97420
1. Our goal is to prevent the introduction of invasive species
arriving at our coast in ballast water.
2. Ideally we don’t want any individual of an invasive species
to reach the coast, but
3. Really what we need to worry about is the establishment
of self-sustaining populations of invasive species.
So what does it mean for a population to be
Self-sustaining?
First, most organisms reproduce sexually. They have to be
close enough together to mate. If they settle too far apart,
they may survive and grow, but they will not reproduce.
Obviously, the population is not sustained. This is the Allee
effect.
It is not enough that organisms get to shore, they have to
settle at densities high enough to over come the Allee effect.
From an oceanographic perspective, we need to look at
transport mechanisms that carry larvae to shore, but they also
probably have to concentrate them at the same time.
Second, to be self-sustaining, the larval phase has to work
- many larvae are pelagic. When their pelagic
development ends they return and settle back into the
adult population. There must be a successful migration
between habitats.
1. Most of the organisms we are dealing with are estuarine.
2. Many are estuarine dependent and the entire life cycle adult and larvae - is confined to the estuary.
3. Others are estuarine dependent, but the larval stage is
exported, development occurs over the shelf, and the larvae
have to migrate back to an estuary to complete the life cycle.
4. Some of the organisms will be coastal and their development
will be similar to the estuarine dependent species with
larvae developing away from the shore and then migrating
back to shore to settle.
•Larvae do not have to behave like water.
•Larvae are generally not passive tracers.
•Larvae swim.
•Few larvae swim as fast as horizontal currents, but essentially all
swim at least as fast as vertical currents.
•Many can make extensive vertical migrations.
•Larvae can control their horizontal movement, but they do it by
controlling their depth - they act like balloonists.
•When we look at how oceanography may affect the
movement of larvae we have to think about the
possible coupled affects of oceanography and larval
swimming behavior.
•Larvae of species that complete their life cycle within an
estuary will likely display behaviors designed to retain them in
an estuary (e.g., tidally timed vertical migrations).
•Coastal and open ocean oceanography are very different from
that in an estuary.
•If these larvae are released in the ocean outside an estuary
they will likely behave incorrectly given the oceanographic
context and fail to migrate to shore.
•The larvae of many species may even be killed by ocean
salinity.
•What about the larvae of estuarine dependent species whose
larvae are exported from the estuary, develop over the shelf
and then migrate back to the estuary (e.g., blue crab).
•Or coastal species whose larvae develop in the coastal ocean
and then migrate back to shore (e.g., green and lined shore
crabs).
•To complete their life cycle, the larvae of these species will
probably have evolved behaviors that exploit coastal
oceanography to increase their chances of returning to shore.
•There are a number of features of coastal oceanography that
are universal - they are present on nearly all coasts.
•These types of larvae are the most likely to migrate to shore
from an offshore ballast water exchange.
Onshore larval transport over the continental
shelf is dominated by the:
1. The internal tides - (a) large internal waves
(solitons) and (b) broken internal waves or
internal bores
2. Upwelling relaxation events.
The Internal Tide
Large internal waves
ride on any density
interface in the water
column.
The waves are non-linear
or solitons. The
propagating waves
generate currents above
and below the wave..
Large tidally generated internal waves can both transport and
concentrate larvae.
Waves propagate at 50 to 20 cm/sec - waves can cross the
shelf in less than a day.
They travel from the shelf break all the way to the beach, and
larvae are transported all the way into the surf zone.
Large internal waves of
expression can break
forming an internal bore.
These transport water
shoreward.
Larvae can be
transported
shoreward within the
bore. The bore
ultimately hits shore
so larvae in the bore
may be transported
to the beach.
Upwelling Fronts Relaxing to Shore
Lucifer faxoni
Blue crab megalopae
Bivalve
larvae
Spionid polychaete
larvae
Over the continental shelf, there are at least three
mechanisms of onshore larval transport that are both
apparently pretty efficient and that will concentrate larvae.
The currents causing transport are present on essentially
all continentals shelves the world over.
My Conclusion:
Because of the higher probability of shoreward larval
transport over the shelf, ballast water should not be
exchanged anywhere over the continental shelf.
Flow in the Southern California Bight is characterized by
numerous eddies, which can retain larvae. In addition, tidal
currents flowing around the Channel Islands and associated
banks generate internal tidal waves that propagate across the
Bight to shore.
Throughout the Bight, there appears to be good mechanisms of
retention and shoreward larval transport.
The Southern California Bight is a poor location for ballast water
exchange.
Onshore transport from the Open Ocean to the
Shelf or Shore:
1.
2.
3.
4.
Classic wind driven upwelling.
Wind driven surface currents (e.g., Langmuir Cells)
Upwelling jets and eddies.
Movement of large estuarine plumes.
In areas that experience sustained upwelling favorable
winds, the source of the upwelled waters is the
continental slope.
Vertically migrating animals found off the continental shelf
and over the slope may enter these waters and be drawn
up onto the shelf during upwelling.
How deep would larvae have to migrate?
Probably somewhere between 50 and 150 meters.
Are larvae strong enough swimmers to make this
migration?
Many crustacean larvae have swimming speeds > 2
cm/sec. They can swim to 100 m depth in a few hrs.
Do larvae swim so deep?
Yes, larvae of a variety of species do.
For example, larvae of the Dungeness crab make a diurnal
vertical migration to > 70 m depth and it looks like the
upwelling generated by the spring transition transports
their larvae onto the shelf from the open sea.
When the wind blows on water it generates Langmuir circulation cells.
Larvae or flotsam at the surface in Langmuir cells move just about
down wind and at about 3% of the wind speed.
These winds may
transport larvae toward
shore.
To be transported, the
larvae just have to
remain at or near the
surface.
In the winter, sustained
winds from the west
push Vellela vellela and
glass fishing floats from
the open ocean all the
way to the coast.
As the upwelling season progresses, perturbations develop in the
coastal currents. These take the form of jets and eddies.
Larvae may be carried
away from shore by
jets and onto the shelf
by eddies.
Unfortunately, the
location of jets and
eddies varies in time
and space making it
difficult to design
ABWEA to avoid their
affect.
Estuarine Plumes as Larval Transporters
wind
Plume
Upwelling winds push an
estuarine plume away
from shore. Large plumes
(e.g., Columbia River and
San Francisco Bay) can
extend far offshore.
Plume
Downwelling winds push
the plume shoreward and
up against the coast where
it travels north hugging the
shore.
wind
Larvae from estuarine species released at sea into plume water
during ballast water exchange may perceive the plume as an estuary.
If they behave as if they are in an estuary (e.g., swim up to
avoid the high salinity water) they could remain concentrated at the
surface in the plume.
When the plume is pushed back to shore during downwelling,
the larvae may be transported all the way to the coast where they will
be in an ideal location to be pulled into an estuary by the flood tide.
My Conclusion:
1. If ballast water contains larvae that vertically migrate,
do not exchange water over the continental slope.
2. I am not sure what to recommend with respect to wind
driven surface transport.
3. Because we cannot predict the location of
topographically generated jets and eddies it is difficult
to make general recommendations.
4. Do not exchange water near any location where
estuarine plumes may be present. This would hold
whether the plume was located off the shelf or on the
shelf.
Current Regulations:
Exchange beyond 200 nm miles.
Good distance - nicely conservative and should do the trick.
Exchange beyond 50 nm and at least 200 m depth.
The distance is OK, probably safe in nearly all cases. I would increase the
depth to 1000 m.
Why?
First, because at 200 m the exchange is right next to the shelf
where it would be easy for them to be transported onto the shelf.
Second, organisms that settle at this location settle into deep
water where they will be unlikely to survive.
Washington
For ABWEA I would suggest:
Yellow - Avoid ballast water
exchange over the continental
shelf and slope. Further, avoid
exchange within the Southern
California Bight.
Blue - Avoid ballast water
exchange adjacent to large
estuaries.
Oregon
California