The Seascapes

The Seascapes

Friday, September 3, 2010

Seascapes, Landscapes & Marine Habitat Dynamics

Because the density and viscosity of water are higher than air, the structures in the ocean span shorter distances and last longer than similar structures in the atmosphere. The space-time diagram above shows that the speed of the ocean is about 100 times slower than the atmosphere. This difference in the speed of the environment on land and in the sea has allowed marine animals to remain more tightly coupled to the oceans “weather” than land organisms are to the atmospheres weather. In the diagram V1-V5 are lines of equal velocity at 103, 30, 0.3 , 3x10-3 & 3x10-5 centimeters per second. The red lines and blue lines are characteristic velocities of the atmospheric and ocean structures that are labeled in the same colors. The approximate space-time scales of forests on land and phytoplankton in the sea are also in the plot. Phytoplankton live and die fast like cavalier poets. The darker dotted line at the bottom is the threshold where laminar flow becomes turbulent flow. Life happens in turbulent flows. The diagram combines information from those in Mamayev (1996) and Steele & Henderson (1994)

One of the really interesting things to think about as a marine scientist is how different from landscapes, seascapes must be from an organisms perspective. Wrestling with this is central to our research program which is concerned with understanding why some parts of the ocean are essential to the survival of marine organisms and the resilience of marine ecosystems. Understanding the nature of the seascape is not easy because we come at the problem with terrestrial bias's; biological and ecological bias's that are even harder to overcome than cultural biases. Understanding differences between marine and terrestrial animals and the environments they occupy is also difficult because it requires a deep understanding of differences in the rates of change in space and time of the external environmental characteristics that affect survival and which therefor have guided the evolution of the animals in the sea. This is a problem of scaling. The space-time diagram above tries to identify the differences in the length-time scales of important dynamic features of the ocean and atmosphere.

Variations in wind, temperature, and precipitation associated with atmospheric storms, fronts, cyclones and long fronts are translated across the surface of the sea to create the waves, fronts, eddies, gyres, and deep ocean circulation which are the “weather” of the ocean. The high density and viscosity of water, its capacity to retain heat and dissolve salts, and its huge volume in the sea, causes the variability in atmosphere to be dampened and slowed down as it is translated across the sea surface. In the plot above, the turbulent structures of the oceans (in blue) are connected by the dotted blue line that is parallel to, but shifted to the right of the structures making the weather of the atmosphere. That rightward shift indicates that it takes much longer for similar structures move over a given distance in the sea. The take away message for me is that while life happens everywhere in turbulent flows and the speed of the ocean and its habitats is about 100 times slower than the speed of the atmosphere.

Because the variability of the atmosphere is slowed down and dampened in the sea, most marine organisms have not evolved the elaborate mechanisms of metabolic and physiological regulation required of terrestrial organisms to maintain homeostasis while in contact with a fast, extremely variable atmosphere. This means that most marine animals are more tightly coupled to the dynamics of the ocean's weather than terrestrial organisms are to the atmosphere's. The properties and dynamics of the water in the ocean are therefor critical to defining the habitats of marine organisms; even those strongly associated with the bottom. There are all sorts of interesting ramifications to this for marine animals who usually start out floating about in the ocean as fertilized eggs a few millimeters long, but grow in spatially dynamic universe made of structured water with 3 spatial dimensions over a huge range of body sizes. The habitat of a baby fish is probably not even perceived by a juvenile or adult. This is not the case for terrestrial organisms.

However there may be some bad ramifications to all this too. Below is a figure made from Sorte et al. (2010) who demonstrated that recent poleward shifts in distributions of marine organisms have occurred at 10 times the speed of the poleward shifts of land animals. These fast species range shifts in the sea may have to do with the tight physiological coupling of marine animals with oceans "weather:. They might also be related to the fact that the dominant force controlling movements in the sea is viscosity instead of gravity which controls the movements of organisms on land. The flows of materials and thus the connections between distant parts of seascapes are greater than for landscapes. Its generally easier to disperse faster over longer distances in the sea. The range shifts of marine and land animals depicted in the graph are probably the result of global climate change.

Steele JH & EW Henderson (1994) Coupling between physical and biological scales. Phil. Trans. R. Soc. Lond. B. 343: 55-9

Mamayev OI (1996) On space time scales of oceanic and atmospheric processes. Oceanology 35(6) 731-734

Sorte et al. (2010)  Marine range shifts and specie introductions: comparative spread rates and community impacts.  Global Ecology and Biogeography.  19: 303-316

No comments: