The application of remote sensing techniques to the study of eutrophication in natural waters and the location and characterization of fronts is considered. The specific problem to be studied is examined along with the feasibility and capabability of remote sensing techniques for each application

Ditoro, D. M.

Iverson, R. L.

Mccarthy, J. J.

English

NASA Technical Reports Server

II. BIOPROCESSES 
Dominic M. DiToro, Richard L. Iverson, 
and James J. McCarthy 
A. Introduction 
Remote sensing of the hydrosphere is a technical discipline which has 
developed to a point where it can now contribute to the solution of important 
scientific and environmental problems. There is neither a lack of problems nor 
a lack of measurement requirements and possible applications of present or pro- 
jected remote sensing capability. This is particularly true in physical, chemi- 
cal, and biological investigations of large water bodies which are invariably 
limited by lack of data. In choosing our recommendations of first-order prob- 
lems to which remote sensing should be applied we have been guided by the fol- 
lowing criteria: 
(a) The importance of the problem as indicated by present level of scien- 
tific and engineering effort being expended in improving our under- 
standing of its causes and of developing methods for controlling its 
consequences 
(b) The unique capability of remote sensing to provide synoptic large 
spatial scale and short temporal scale data as compared to conven- 
tional ship-based sampling procedures 
(c) The parameters available for measurement using present and projected 
remote sensing techniques 
(d) The demonstrated need for these measurements in ongoing and planned 
investigations. 
We recommend two classes of problems be considered which are related in 
certain respects but distinct in emphasis: (1) eutrophication and (2) location 
and characterization of fronts. 
B. Eutrophication 
1. Statement of the Problem 
(a) Why It Is Important Scientifically and Politically 
Artificially accelerated eutrophication in natural waters is a manifesta- 
tion of man's age-old practice of disposing of his wastes in the most convenient 
river, lake, or ocean. This process, which is often referred to as cultural 
eutrophication, can result from the perturbation of the environment which 
(1) stimulates the production of algae without a concomitant increase in crop- 
ping or other loss processes, (2) stimulates certain nuisance algae to out- 
compete other more desirable algae, or (3) suppresses natural grazing processes. 
The result of extreme cultural eutrophication is an accumulation of algae which 
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are classed as undesirable because they result in a disruption of the normal 
food web. The herbivores do not accept them as suitable food substitutes for 
the algae which have been displaced, and with advanced eutrophication the water 
body usually loses important finfish and shell fish populations. The resultant 
accumulation of algae depletes nutrients from the water, the algae become light 
limited, and subsequently decompose. The decaying algae are an aesthetic prob- 
lem on beaches, they affect the taste and odor qualities of the water supply, 
and when in high populations, the bacterially mediated decomposition can result 
in reduced oxygen concentrations and widespread fish mortality. 
Cases of cultural eutrophication have been thoroughly documented for pol- 
luted natural waters, and corrective measures have successfully reversed the 
development of eutrophication in some instances, primarily small freshwater 
lakes. However, the remedial measures for the eutrophication of large water 
bodies are extremely costly and the projected degree of their effectiveness is 
still a matter of debate. It is these problems that are currently of major 
importance. 
The first biological indication of cultural eutrophication is a change in 
either biomass or the species composition of the algae, the organisms at the 
base of the aquatic food web. Oceanographers and limnologists are often unable 
to recognize such changes in the biota until they have reached fairly dramatic 
levels. Consequently the scientist usually becomes aware of a developed 
eutropic condition or a large population bloom (such as red tides or the 
unusual dinoflagellate dominance and dissolved oxygen depletion in the New York 
Bight during the summer of 1976) after it has developed. At the heart of this 
problem is our inability to synoptically monitor large areas for quantity and 
quality of algal biomass. 
(b) Consumers of Results 
Information regarding eutrophication in both marine and fresh waters is of 
great use to scientists interested in factors which regulate both biological 
productivity and community structure. Surveillance of the Great Lakes is cur- 
rently being carried out by a joint U.S.-Canadian effort and large-scale control 
programs are being designed. Investigations of coastal water eutrophication in 
the New York Bight by the MESA program of NOAA and in the Chesapeake Bay by EPA 
are currently underway. NASA remote sensing data can complement these efforts 
and add significantly to the available data. This information is needed in 
order to make decisions regarding the management of pollutant inputs, fishery 
resources, and recreational uses of the natural waters. Academic scientists 
who are interested in understanding the process of eutrophication would also be 
major users of these large data sets of environmental parameters which influ- 
ence productivity. 
2. What Needs To Be Measured? 
(a) Limitation of Existing Conventional Techniques 
The parameters which must be measured are the amount of biomass in the 
water column, its species composition, its rate of production, numerous environ- 
mental factors which regulate productivity (light, temperature, salinity, and 
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nutrients), and, most importantly, the time and space scales of the variability 
in these features. All of the above can be measured from stationary ships, and 
in most instances the analytical precision is adequate. 
The major limitations to conventional approaches are: (1) the measurements 
are not all available in real time, (2) even with multiple ship operations, the 
restricted spatial coverage limits the ability to generalize about the overall 
study area, and (3) no information is available on the behavior of the study 
area during the intervals between sampling periods. The scales of time which 
are important in eutrophication can be as small as days since the organisms 
replicate in periods of this length. But of equal importance is the longer 
term view which may range from a few to several years. Spatial scales of 
importance range from a few to hundreds of kilometers. The observations made 
from shipboard actually characterize only a small volume of water, and whereas 
we know that we can generalize from such observations to areas of hundreds of 
square kilometers in the open ocean, we are limited to areas of single kilo- 
meters or less in physically dynamic coastal regions of oceans and large lakes. 
Large-scale variability, even in the open waters of the Great Lakes, has also 
been observed during intensive lake-wide sampling (e.g., during the Inter- 
national Field Year investigations of Lake Ontario). To a large degree, the 
present limitations stem from the need for spending a large part of the day on 
station in order to properly characterize the properties mentioned above and 
the transit time between stations. 
(b) Feasibility and Capability of Remote Sensing Techniques 
Remotely sensed synoptic sea surface temperature, light penetration, 
surface currents, and quantity and quality of plant pigments at a daily fre- 
quency for study areas of interest would yield the data which, when combined 
with shipboard measurements of'the other parameters of importance, would 
greatly improve the understanding of this problem which is otherwise difficult. 
We are now at the point where the development and verification of sound models 
for eutrophication are essential if we are to progress, and remote sensing is a 
key component in such an endeavor in large lakes and coastal waters. 
3. Recommendations 
Of prime importance in a NASA effort to apply remote sensing to the prob- 
lem of eutrophication is a solid interfacing with other governmental agencies 
which are conducting scientific field programs which address this problem. 
Such interagency limnological and oceanographic investigations would provide 
the proper focus in addition to the shipboard observational and experimental 
capabilities. They must also have a strong modeling component which would 
greatly benefit from synoptic data which remote sensing can generate. 
NASA should also support efforts to improve knowledge of optical physics 
in order to further develop the potential of both active and passive sensing 
systems. A program to assess the degree to which existing technology can pro- 
vide accurate estimates of the depth of light penetration, the quantity and 
quality of plant pigments in waters of different depths, and in waters with 
different amounts of nonplant particulate material, and other variables is 
required. 
C. Location and Characterization of Fronts 
1. Statement of the Problem 
(a) Why It Is Important Scientifically and Politically 
Fronts are features characterized by sharp gradients in physical and bio- 
chemical variables in aquatic environments. The length scales of fronts range 
from lo2 meters for some estuarine plume fronts to lo6 meters for some conti- 
nental shelf fronts. Fronts persist on the order of hours for some tidal fronts 
to years for the shelf break front in the Bering Sea. Factors which interact 
to cause frontogenesis are not well understood; therefore frontogenesis is cur- 
rently an active research area in physical oceanography. Salinity gradients 
associated with shelf break fronts suggest that a balance between freshwater 
input from rivers along the northeast coast of North America and in the Bering 
Sea and offshore oceanic water is important in frontogenesis. The tides are 
known to be a primary source of energy for frontogenesis in shelf waters 
around the British Islands. The M2 internal tide has been strongly implicated 
in frontogenesis of the Nova Scotian shelf break front. All of the evidence to 
date suggests that fronts may be an important site for energy dissipation on 
continental shelves. Research in localized, readily available sites where 
regular physical dynamics occur is very important in attempts to understand the 
dynamics of the upper ocean. 
Major fisheries along the east coast of the North American continent and 
in the Bering Sea are associated with continental shelf fronts. Enhanced 
dynamics of phytoplankton and zooplankton have been reported in fronts relative 
to dynamics in shelf waters not influenced by fronts. Distinct pelagic and 
benthic food webs appear to be associated with different regions along a cross- 
shelf transect through several fronts in the Bering Sea shelf waters. These 
food webs are coupled to the fronts which are stable features throughout the 
spring and summer. The fronts afford specific sites in which to investigate 
dynamics of significantly different food webs. Factors which control nutrient 
input to the photic zone in fronts are poorly understood, although the M2 
internal tide may drive vertical advection of water in the Scotian shelf break 
front. 
The political significance of fronts is coupled with the tremendous bio- 
logical productivity of fronts. Foreign fishing vessels are currently the pri- 
mary vessels working fronts along the continental shelves of the United States. 
Management of U.S. coastal fisheries along the Atlantic coast and in the Bering 
Sea will be primarily focused on fronts in the near future. 
(b) Consumers of Results 
There are a number of investigations of frontal processes which would bene- 
fit now or in the next several years from application of NASA technology to 
location and characterization of mesoscale features of fronts. Some of these 
investigations include PROBES, a study of the Bering Sea funded by NSF Polar 
Sciences, DOE-funded investigations of the Gulf Stream front along the south- 
east Atlantic shelf, NMFS investigations of fronts near Georges Banks, and vari- 
ous smaller projects funded by NSF which are addressing front processes in the 
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New York Bight. NASA technology could be used to assist NME'S in monitoring the 
activity of fishing vessels in fronts along the continental shelves of the 
United States. 
2. What Needs To Be Measured 
(a) Limitations of Existing Conventional Techniques 
Ships are the primary platforms which have been used in research on fronts, 
although some visible spectrum photography from light aircraft and satel.lites 
has been used to show spatial scales of fronts. Ships are obviously limited in 
the distance over which samples can be taken in short periods of time. This 
creates serious problems.both in characterizing the spatial scales of large 
fronts and in obtaining even quasi-synoptic data sets. Large-scale motion 
associated with fronts is not very well known primarily as a consequence of 
limitations in shipboard data collection. Shelf break fronts appear to oscil- 
late with the tides. Large-scale waves have been observed in shelf break fronts 
along the northeastern U.S. coast. Events of time scale 2 to 4 days appear to 
be important in frontolysis near canyons along the southeastern U.S. Atlantic 
coast. The temporal response of fronts to transient forcing events such as 
wind stress is largely unknown because ship scheduling is not responsive to the 
need to study transient events. 
(b) Feasibility and Capability of Remote Sensing Techniques 
Remote sensing of surface temperature, salinity, color, reflectance, and 
roughness would provide a set of data from which to obtain estimates of the 
spatial scales and near-surface dynamical features of fronts on all scales. 
The spatial resolution required for different variables depends on the nature 
of the front under investigation. For example, a resolution of 100 m would be 
adequate for characterizing the fronts in the Bering Sea or along the Atlantic 
coast of North America. Finer levels of resolution would be required for the 
study of plume fronts associated with the Mississippi or Columbia Rivers. The 
laser methods for measuring turbidity and chlorophyll now under development by 
NASA would allow finer resolution of features within fronts. Laser data would 
presumably be available more rapidly than would data collected by satellite, 
a consideration which is important when remote and surface data collection for 
real-time experiments is planned. 
3. Recommendations 
(1) NASA personnel should interact with investigators actively involved in 
frontal research so that information about front characteristics, data require- 
ments, and actual and potential instrument capability can be exchanged in a 
timely manner. 
(2) NASA should allocate resources to examine Coastal Zone Color Scanner 
(CZCS) data for evidence of fronts in locations where their existence is sus- 
pected based either on empirical or theoretical bases. This effort would deter- 
mine the length scales of biologically active fronts on continental shelves 
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where large gradients in chlorophyll occur. Time series observations should be 
conducted for selected fronts to characterize the seasonal pattern of changes 
in frontal chlorophyll signature. 
(3) NASA should interact with programs which are now addressing fronto- 
genesis so that ground truth can be coupled with CZCS imagery in an attempt to 
elucidate forcing variable frequency and frontal response between periods when 
ships are on station. The input of NASA will be critical in attempts to under- 
stand the time scales of frontogenesis and frontolysis. 
(4) NASA should continue the development of laser remote sensing instru- 
ments. Development should proceed in concert with field tests of the instru- 
ments in locations where frontal research is being performed. This would allow 
feedback between instrument development and application so that the laser 
methods are optimized for the variables of interest. When instrument relia- 
bility and performance are satisfactory, the instruments should be programmed 
into ongoing and planned research on fronts. 
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