Coastal water darkening – consequences
for coastal ecosystems
Konsekvenser av mørkere kystvann for kystøkosystemer
Forskerseminar 24. oktober 2012, Havet og kysten
DL Aksnes
University of Bergen
Outline
• The Norwegian Coastal Water
• Coastal water darkening
– Sources for attenuation of light
– Expected effects
• Future work
The Norwegian Coastal Water (NCW)
Hjort og Gran (1899)
• Less saline than oceanic
water
• Contains freshwater that
has drained to the Baltic
Sea, southern North Sea ,
and from the Norwegian
coast
• Moves from south to
north (Norwegian Coastal
Current, NCC)
• Freshwater contains more
Color Dissolved Organic
Matter (CDOM) than
oceanic water
Coastal Water
• Sources for attenuation of light
Lurefjorden
Masfjorden
Light(depth) = Light(surface) e-K*depth
K =K(water) + K(algae) + K(CDOM) +...
CDOM: Color Dissolved Organic Matter
(humic substances, gelbstoff)
Coastal Water
Water samples from 40 locations
• Sources for attenuation of light
K  k1  k2 Sal  k3Chl k 4(6  Oxy)
Coefficient,
Estimated value
β
p - value
factor
k1, Intercept
k2, Salinity
k3, Chlorophyll
k4, Oxygen
< 10-5
0.97 ± 0.08
-0.023 ± 0.002
0.046 ± 0.008
-0.022 ± 0.003
-0.50 ± 0.05
0.29 ± 0.05
-0.31 ± 0.04
< 10-5
< 10-5
< 10-5
Coastal Water
• Sources for attenuation of light
Atlantic source
K(CDOM) = 0.05 m-1
Coastal water
K(CDOM) = f(salinity)
+g(oxygen)
Freshwater source
K(CDOM) = 0.97 m-1
Long term trends in water clarity
Deeper than 100 m
Shallower than 100 m
Deeper than 100 m
Shallower than 100 m
Coastal Water darkening – expected effects
• The light attenuation (K) determines euphotic zone
properties
• This can be simulated by standard ecosystem models:
P
P
2P
  ( N , I ) P  mP  v
 2
t
z
z
N
2 N
  ( N , I ) P  mP   2
t
z
 ( N , I )  max min(
I
N
,
)
Hi  I H n  N
z



I  I in exp   K bg z  k  P(t ,  )d 
0


Phytoplankton
Nutrient
Nutrient and light limitation
Light at depth
Coastal Water darkening – expected effects
• The light attenuation (K) determines euphotic zone
properties
Coastal Water darkening – expected effects
• The light attenuation (K) determines euphotic zone
properties
Clear
water
Phytoplankton
Nutrient
Dark
water
This looks like eutrophication
although nutrient input is
unchanged!
Coastal Water darkening – expected effects
Habitat becomes shallower,
but also narrower
Visual predation becomes less
efficient
Coastal Water darkening – expected effects
• Analysis of fish and jelly competition in the Baltic Sea
Water
clarity
In press, Mar Ecol Prog Ser
Coastal Water darkening – expected effects
• Analysis of fish and jelly competition in the Baltic Sea
Today
In press, Mar Ecol Prog Ser
Pristine situation
Coastal Water darkening – expected effects
o Reduced water clarity
o Shallower and narrower euphotic zone
o Increased phytoplankton and nutrient concentrations
in surface layers
o Reduced sublittoral habitat (e.g. kelp and associated
fauna/fishes)
o Reduced abundance of visual predators (fish) and
increased abundance of tactile predators (jellyfish)
 Eutrophication symptoms despite no increase in
nutrient input
 Combination of nutrient input and darkening is likely
to magnify symptoms
Future work
• To understand changes in NCW optics as a result of
changes in terrestrial and limnetic systems
Future work
• To understand changes in NCW optics as a result of
changes in terrestrial and limnetic systems
Future work
• To understand changes in NCW optics as a result of
changes in terrestrial and limnetic systems
Will enable revision and increased realism of applied marine ecosystem
models like the “NORWECOM” mode system
P
P
2P
  ( N , I ) P  mP  v
 2
t
z
z
N
2 N
  ( N , I ) P  mP   2
t
z
 ( N , I )  max min(
I
N
,
)
Hi  I H n  N
z


I  I in exp   K bg z  k  P(t ,  )d 
0


Research proposal has
been sent to “Havet og Kysten”
Future work
• Analyses of long term changes in coastal ecosystems
should not ignore changes in the optical environment!
Nedgang i kysttorsk
Tap av tare habitat
Økte manetforekomster
…..etc.
Eutrofi eller ikke?
Future work
• Monitoring of NCW optical properties are welcome
(scattered Secchi observations are useful but not
sufficient!)