National Aeronautics and Space Administration

Wallops Flight Facility

Laser Biomonitoring

ADVANCED COASTAL LASER BIOMONITORING

Coastal and estuarine waters are highly productive and complex ecosystems, but efforts to examine them on large spatial and temporal scales are largely limited to remotely-sensed estimates of Chl a biomass. An advanced pump-and-probe (P&P) airborne laser technology has been recently developed at NASA Goddard Space Flight Center. The P&P system provides remote measurement of important phytoplankton photosynthetic variables, such as the functional absorption cross-section of photosystem II (PSII), PSII photochemical efficiency, PSII turnover time, the rate parameters of singlet-singlet and singlet-triplet annihilations, and carotenoid triplet lifetime along with pigment and organic matter fluorescence, down-welling and upwelling hyperspectral measurements and IR surface temperature. The utilization of an airborne platform provides the potential for rapid remote characterization of phytoplankton photosynthetic activity, biomass and diversity over large aquatic areas at synoptic space/time scales.

The P&P LIDAR technique is one of the first practical implementations of ‘superactive’ remote sensing . The distinguishing feature of this new class of technique is the ability to remotely cause desirable changes in the subject’s functionality to retrieve additional information unattainable with any other passive or active techniques. The P&P technology may be complimented by recent developments in assessments of phytoplankton taxonomic variability from airborne LIDAR measurement. This research is primarily focused on multicolor laser excitation of Chl, PUB, PEB and phycocyanin fluorescence bands to remotely implement fluorescence excitation technique. A laboratory prototype of the Laser Phytoplankton Analyzer (LPA) shipboard system has been successfully tested with representative sets of phytoplankton cultures and their mixtures.

Both P&P and LPA systems provide new unique capabilities for advanced laser biomonitoring. Below we present examples of the initial utilization of the P&P technique in the US coastal and near-shore areas. Potential of the LPA technology for improved taxonomic, pigment and physiological analysis of phytoplankton is discussed.

Present Activities:

Coastal Biomonitoring with an Airborne P&P LIDAR:

In October 2000, we initiated airborne LIDAR surveys to study dynamics and/or spatial variability of phytoplankton photochemical characteristics, pigment and CDOM fluorescence in coastal areas of the Middle Atlantic Bight, in the Chesapeake Bay, Delaware Bay and Pamlico Sounds (NC). An example of such a survey around the Delmarva Peninsula is presented in Fig. 1. Only 3 hours were required to acquire the informative high-resolution data set for detailed characterization of the area. Generally, Atlantic coastal areas of Delmarva Peninsula were found to be less productive (see ~10 times lower Chl and phycoerythrin content) than respective areas in the middle/upper Chesapeake and Delaware bays. Distributions of Chl were in general agreement with SeaWiFS Chl data (false colors in the left corner insert), but provided more detailed information about spatial variability in relation to important taxonomic and environmental characteristics such as phycoerythrin, CDOM and surface temperature. Photochemical efficiency in upper Delaware Bay was high (~0.65) and comparable to levels found under ideal conditions in the laboratory indicating that the population was in robust physiological condition at the time. This is consistent with early bloom development in Delaware Bay occurring in response to increased light-levels under nutrient-replete conditions as stratification develops. Monitoring the effects of light- and nutrient status on a wide array of taxa would allow an informed interpretation when the data on natural populations is collected synchronously with that on proxies for the physical environment (temperature, attenuation as an indicator of resuspension and sediment transport, CDOM as a tracer of freshwater inputs, etc.). We have already conducted 4 all-around P&P LIDAR surveys in October 2000, March 2001 (2 surveys) and March 2002. Acquired data provide potential for studying dynamics of spatial variability in bio-environmental characteristics over a range of temporal scales, including seasonal.

Figure 1. P&P LIDAR biomonitoring of coastal spring bloom in coastal area of the Middle-Atlantic Bight

The P&P LIDAR, with its unique combination of superactive, active, and passive (SAP) sensors and the utilization of the airborne platform, provides new possibilities for monitoring coastal processes over large aquatic areas, including interactions between physical and biological structures. When combined with satellite information, the SAP technique can be turned into a powerful research approach for coastal process studies. For example, we present in Fig. 2 results of our P&P LIDAR measurements in the area of temperature front (coastal zone of the Middle-Atlantic Bight, April 9, 2001). Chl maxima along the LIDAR transect were generally located in the areas of sharp temperature gradients (compare left and right upper panels in Fig. 2, respectively). The phycoerythrin transect distribution indicates similar trends (compare upper and lower left panels in Fig. 2). Analysis of Phycoerythrin/Chl fluorescence ratio allows retrieval of some general information about taxonomic composition of phytoplankton community.

Figure 2. An example of the P&P LIDAR survey around Delmarva Peninsula. March 31, 2001

The P&P LIDAR technique has the potential to improve remote estimates of productivity because it provides direct high-resolution measurements of the photochemical characteristics of photosystem II (PSII). We have recently introduced a new parameter, ‘photochemical productivity potential’,

PPP = σPSII*ΦPSII*τPSII-1*Chl .

By definition, it is a product of PSII functional absorption cross-section, σPSII, its photochemical efficiency, ΦPSII, PSII turnover rate, τPSII-1, and Chl concentration. Multiplied by the ambient irradiance and several constants, the PPP magnitude can be used for assessment of the maximum potential rate of gross photosynthetic oxygen evolution and/or carbon assimilation (primary productivity). For evaluation purposes, we present in the lower right panel of Fig. 2 the PPP magnitude distribution retrieved from P&P LIDAR measurements. Generally good spatial correlation between the Chl and PPP data sets (compare right upper and lower panels in Fig. 2) confirms that satellite Chl data can provide a reasonable basis for assessment of biological productivity. On the other hand, incorporation of phytoplankton photochemical parameters critical for phytoplankton photosynthetic performance should significantly increase the accuracy of productivity estimates.

During our Spring 2002 field campaign, we initiated 2D P&P LIDAR biomonitoring in the coastal areas of Middle-Atlantic Bight. The comprehensive set of photosynthetic characteristics provides for fast detailed remote characterization of bio-environmental situation in the area. An example of such measurements conducted during another 3-hour survey of phytoplankton Spring bloom in the Delaware Bay is displayed in Fig. 3. Though we found generally good spatial correlation between the LIDAR (green line) and SeaWiFS (see the insert in lower left corner) Chl distributions, the LIDAR data provides for more detailed characterization in the coastal area. Similar spatial patterns of phycoerythrin and PSII functional absorption cross-section suggest that the latter variability was mostly driven by taxonomic changes in the area.

Figure 3. 2D mapping of phytoplankton photosynthetic and fluorescence characteristics during with a P&P LIDAR. Delaware Bay. March 23, 2002

As in the Fig.1, the maximal magnitudes of phytoplankton photochemical efficiency (Fv/Fm ~ 0.65) were observed in the upper Delaware Bay. This indicates excellent physiological and functional state of phytoplankton caused by nutrient-replete conditions. We plan to compare the airborne LIDAR data with results of a shipboard survey conducted in this area in parallel with LIDAR measurements. Current remote-sensing estimates of phytoplankton biomass using Chl are based largely on water-leaving radiance in the blue and green (443, 490, 510 and 555 nm). In Case II waters, retrieving the signal due to Chl is complicated by the high and variable contributions of detritus and chromophoric dissolved organic material (CDOM) at these wavelengths. As an illustration (see Fig. 4), we compare SeaWiFS and LIDAR P&P estimates of Chl made in Spring 2001. The dark line across the SeaWiFS Chl image (upper right corner) represents the LIDAR transect. In the middle of the transect, where there was relatively low dissolved organic matter (CDOM, blue line in the panel), we observed reasonably good agreement between Chl assessment by SeaWiFS and LIDAR. In areas of high organic matter concentrations, SeaWiFS’ estimates of Chl were several times higher than the LIDAR ones.

Figure 4. An example of P&P LIDAR validation of SeaWiFS chlorophyll data in the coastal and offshore area of the Middle-Atlantic Bight

Although we did not have direct shipboard validation of LIDAR data along this transect, the generally high correlation between LIDAR and shipboard Chl measurements in case II waters (e.g., r =0.92) obtained at the same time during our NASA/NOAA LIDAR survey of a spring bloom along the New Jersey coast suggests that there was likely a significant overestimation of Chl by the satellite in the transect areas with high CDOM. During our Spring 2001 campaign, we acquired initial LIDAR validation data of satellite Chl assessment in coastal case II waters.

Towards Shipboard and Airborne Laser Analysisof Phytoplankton in the US Coastal Areas:

The unique combination of sensors and retrieved characteristics of the airborne superactive-active-passive (SAP) LIDAR also provides new taxonomic analytical possibilities. For example, Fig. 5 displays the data set acquired with the P&P LIDAR along 300-kilometer transect between Delmarva Peninsula and Long Island in March 1999. The light blue line represents water Raman scattering measured at 650 nm, the green one displays chlorophyll fluorescence at 685 nm. The dark blue light at the bottom is phytoplankton photochemical efficiency inhibited by ambient light (superactive data set). The most remarkable features of the transect are three patches of phycoerythrin-containing cyanobacteria (marked 1, 2 and 3 in Fig. 5), which is obvious from the yellowish lines displaying phycoerythrin fluorescence measured at 560 and 590 nm. Complicated correlation pattern of spatial variability in LIDAR signals was observed along the transect (see table in the left lower corner in Fig. 5).

Our current research goal is to implement the combined potential of the new SAP technology to improve taxonomic capabilities of airborne LIDAR remote sensing in the coastal areas. A critical question is to determine the taxonomic level at which the remote technologies can provide reliable analytical results. A multi-wavelength laser excitation of multi-band pigment fluorescence (MLE-MPF) technique was selected as a primary technology for achieving improved taxonomic analysis. This approach is essentially a laser modification of the conventional fluorescence excitation technique, which provides information about spectral absorption of chlorophyll-a and photosynthetic accessory pigments, such as chlorophyll-b, chlorophyll-c, phycobilins and photosynthetic carotenoids. Future airborne LIDAR implementation of the MLE-MPF technique assumes the availability of a powerful multiwavelength laser source for excitation of phytoplankton pigment fluorescence.

Figure 5. An example of variability in LIDAR return signals driven by taxonomic changes

A representative, phylogenetically-diverse set of 15 phytoplankton cultures from 7 taxonomic groups, including Diatoms, Pelagophytes, Prasinophytes, Prymnesiophytes, Rhodophytes, Cryptophytes and Cyanobacteria, were chosen for the initial stage of our laboratory experiments. We acknowledge great biological support from Dr. Hugh MacIntyre (University of Maryland Center for Environmental Science). Special emphasis was placed on conducting extensive validation of the newly developed algorithms and instrumentation using well-established techniques and commercial instruments. A prototype of the shipboard Laser Phytoplankton Analyzer (LPA, see Fig. 6) was designed and built for laboratory verification and optimization of the MLE-MPF technique. At the current stage of the research, 11 of 15 examined cultures could be recognized with the proposed algorithms.

Figure 6. A prototype of the shipboard Laser Phytoplankton Analyzer

Our analysis revealed that, in addition to taxonomic recognition, the LPA technique can be potentially utilized for non-invasive in vivo and in situ concentration assessment of chlorophyll-a and accessory photosynthetic pigments, including chlorophyll-b, chlorophyll-c, and phycobilins. In addition, some general information about overall content of photosynthetic carotenoids may be retrieved. The LPA shipboard and airborne systems will provide new possibilities for advanced laser biomonitoring in coastal and estuarine regions.

Future Directions:

  • Continue operational utilization of the P&P LIDAR technology for process studies, environmental surveys and satellite validation in US coastal regions
  • Develop an advanced NASA Airborne Oceanographic superactive-active-passive (SAP) LIDAR of the 4th generation
  • Develop, validate and operationally utilize a flow-through Laser Pigment Analyzer (LPA) for shipboard pigment, taxonomic, and physiological analysis of phytoplankton in coastal areas
  • Develop and validate an airborne prototype of the airborne LPA system for remote surveying

Collaborators:

Lead Investigator: Alexander M. Chekalyuk