Principal Investigator
I directed the Phytoplankton Laboratory at SERC from 1986 to 2014. The goal of my program was to increase understanding about the relationship of phytoplankton to their environments, and to communicate the findings and their significance to my peers in science, management and the general public. Using the Rhode River and Chesapeake Bay as a focal site, our lab conducted both long-term observational studies and manipulative experiments. We investigated and modeled the causes determining phytoplankton populations and their primary productivity, as well as the effects of phytoplankton on the estuarine environment, particularly water clarity. We also conducted comparative studies at locations around the world that helped generalize and broaden the impact of our findings from the core Chesapeake Bay site.
The primary legacy of the Phytoplankton Laboratory is a well-documented, quality-assured, long-term dataset on phytoplankton dynamics and estuarine optics. The permanent datasets that we assembled consist of continuously monitored water quality variables, weekly to bi-weekly measurements of phytoplankton chlorophyll, photosynthesis-irradiance curves, species composition, and inherent optical properties of the Rhode River and numerous other coastal embayments in the Chesapeake region and other coastal systems.
The completed work on phytoplankton dynamics tells a story of how land-sea interactions at local and regional scales structure and regulate plankton communities and optical water quality in estuaries. Links to additional details and access to data are provided below.
Phytoplankton Photosynthesis & Primary Productivity
In order to understand the environmental factors that govern phytoplankton growth, we must first recognize two processes. Photosynthesis converts light energy into organic carbon compounds. Productivity uses energy and carbon stored in organic carbon, plus nitrogen, phosphorous, and other nutrients, to carry out metabolism and build new tissues.
PHOTOSYNTHESIS
We measured parameters in the photosynthesis-irradiance (P-E) relationship of phytoplankton at weekly to bi-weekly intervals for 20 years at six stations on the Rhode River, Md. We grouped variability in the light-saturated photosynthetic rate (Pbmax) into interannual, seasonal, and spatial components. The seasonal component was greatest, followed by interannual and then spatial. Pbmax correlated most strongly with temperature and the concentration of dissolved inorganic carbon (IC), with lesser correlations among other variables. Spatial variation was relatively weak and uncorrelated with ancillary measurements. The results demonstrated that both the overall distribution of Pbmax and its relationship with environmental correlates vary from year to year. Coefficients in empirical statistical models became stable after accumulating 7 to 10 years of data. The main correlates of Pbmax are amenable to automated monitoring, so that future estimates of primary production can be made without labor intensive incubations.
Publication:
Gallegos, C. L. 2012. Phytoplankton photosynthetic capacity in a shallow estuary: Environmental correlates and interannual variations. Mar. Ecol. Prog. Ser. 463: 23-37, doi:10.3354/meps09850.
Data:
Gallegos, C. L. 2012. Photosynthetic parameters, chlorophyll concentration, and diffuse attenuation coefficients in the Rhode River, Maryland (USA), 1990-2009. PANGAEA, Data Publisher for Earth and Environmental Science, dataset no. 816494. doi:pangaea.de/10.1594/PANGAEA.816494.
PRIMARY PRODUCTIVITY
We calculated daily rates of phytoplankton primary production from measurements of light saturation curves of photosynthesis for 20 years at six stations on the Rhode River, Md. Daily production averaged 1,319 (range 1.4 to 15,800) mg C m‑2 d‑1. The seasonal signal was the greatest source of variation, followed by spatial, then interannual. The seasonal pattern was driven by coinciding summer maxima in both the chlorophyll a biomass (B), and the light-saturated photosynthetic rate normalized to chlorophyll a (PBmax). The spatial pattern was characterized by a region in which production was relatively constant despite declining depth, a station at which production was reduced by truncation of the depth profile of production, and an area where mean production was lowest, but variance was highest, due to local flow causing either localized blooms or washout of biomass and high turbidity at the most up-estuary station. The analytical approach adopted here allowed us to discern these patterns despite a high degree of overall variability, and should be similarly useful in a wide range of systems.
We also analyzed annual primary production in the nutrient-rich Rhode River in relation to climatological and ecological factors. Annual production (PA) averaged 328 g C m‑2 y‑1 (range 152 to 612). Interannual variability was statistically significant, but there were no significant non-random variations over the available 19 years. The magnitude of the spring dinoflagellate bloom and timing of nitrate depletion were both significant predictors of PA. Phytoplankton biomass (B) and light-saturated photosynthetic rate normalized to chlorophyll (PBmax) were of similar magnitude in their influence on the variance in annual production. The high degree of variability in PBmax weakened efforts to model both daily and annual production from measurements of chlorophyll and light attenuation.
In order to evaluate the effectiveness of management efforts to reduce primary production in the Rhode River, 4 to 15 years of measurements of chlorophyll and light attenuation would be needed to detect a change in trophic status of the subestuary, depending on the level of reduction achieved in PA. Average daily production would have to be reduced below 1,052 mg C m-2 d-1 to achieve moderate, healthy nutrient levels in the Rhode River.
Finally, we determined the role of events in generating short-term variability in production, and how they contribute to interannual variability in annual production. We examined residuals from the seasonal and spatial mean daily rates in a 20-year time series of primary production in a nutrient-rich subestuary of Chesapeake Bay. The variance at the event scale was larger than both the seasonal and spatial variance. Residuals were classified as events if they exceeded ±ln(2), signifying a multiplier or divisor of 2 above or below the seasonal-spatial mean. Spatially, events were most frequent at the most upstream station affected by runoff from the local watershed, and temporally most frequent in spring at all stations. The greatest variance was associated with variability in spring due to the occurrence of extremely large spring blooms or their complete absence. The effects of major storms on phytoplankton production in this system were found to be moderated by the relatively short residence time of the system.
Publications:
Gallegos, C. L. 2014a. Long-term variations in primary production in a eutrophic sub-estuary: 1. Seasonal and spatial variability. Mar. Ecol. Prog. Ser. 502: 53-67, doi:10.3354/meps10712.
Gallegos, C. L. 2014b. Long-term variations in primary production in a eutrophic sub-estuary: 2. Interannual variation and modeling. Mar. Ecol. Prog. Ser. 502: 69-83, doi:10.3354/meps10713.
Gallegos, C. L., and P. J. Neale. 2015. Long-term variations in primary production in a eutrophic sub-estuary: Contribution of short term events. Est. Coastal Shelf Sci. 162: 22-34, doi:10.1016/j.ecss.2015.01.015.
Data:
Gallegos, C. L. 2013. Phytoplankton daily production in the Rhode River, Maryland (USA), 1990-2009. PANGAEA: Data Publishers for Earth & Environmental Science, dataset no. 819825. doi:10.1594/PANGAEA.819825
Coastal Optical Properties
My interest in factors governing the penetration of light in coastal waters developed as a natural outgrowth of my interest in phytoplankton photosynthesis and primary production. Coastal waters receive inputs of suspended sediments and colored dissolved matter from land. These materials exert a strong effect on the scattering and absorption of light underwater. Furthermore, the concentrations of these materials may vary independently of the concentration of phytoplankton chlorophyll, making variability of coastal optical properties highly complex.
Understanding optical properties in estuaries is not only important for their role in phytoplankton primary production. The optical properties of water also exert a critical influence on the distribution and abundance of submerged aquatic vegetation (SAV), and are a key component of remote sensing studies. My research on estuarine optics was a significant source of external funding and led to collaborations with international researchers and management communities in the Chesapeake region, Florida, Massachusetts and New Zealand.
OPTICAL PROPERTIES AND SAV HABITAT REQUIREMENTS
Beginning with my involvement with the Chesapeake Bay SAV Technical Synthesis Team in 2000, I researched how to use radiative transfer modeling to determine water quality conditions suitable for submerged aquatic vegetation. Initially, this work focused on spectral light attenuation in the Rhode River and Maryland coastal bays [1, 2]. Starting in 2007, I expanded the scope to include studies in coastal Massachusetts [3], North Carolina [4], Virginia [5], and Belize [6]. In a collaborative project with R. C. Zimmerman (Old Dominion University), we merged this approach with physiologically based models of SAV growth (R. C. Zimmerman) and GIS approaches, successfully demonstrating the ability to predict the distribution of SAV across the submarine landscape as a function of water quality conditions and changing climate drivers (e.g. temperature, CO2 and sea level [5]). This project advanced the science of ecosystem-based approaches for coastal management by improving our ability to model and predict the sensitivity of SAV communities to natural and anthropogenic disturbances. Merging the models aids managers by eliminating the requirement for independent estimates of SAV light requirements.
Publications:
[1] Gallegos, C. L., D. L. Correll, and J. W. Pierce. 1990. Modeling spectral diffuse attenuation, absorption, and scattering coefficients in a turbid estuary. Limnol. Oceanogr. 35: 1486-1502,doi:10.4319/lo.1990.35.5.1486.
[2] Gallegos, C. L. 2001. Calculating optical water quality targets to restore and protect submersed aquatic vegetation: Overcoming problems in partitioning the diffuse attenuation coefficient for photosynthetically active radiation. Estuaries 24: 381-397, doi:10.2307/1353240.
[3] Kenworthy, W. J., C. L. Gallegos, C. Costello, D. Field, and G. Di Carlo. 2014. Dependence of eelgrass (Zostera marina) light requirements on sediment organic matter in Massachusetts coastal bays: implications for remediation and restoration. Marine Pollution Bulletin 83: 446-457, doi:10.1016/j.marpolbul.2013.11.006.
[4] Biber, P. D., C. L. Gallegos, and W. J. Kenworthy. 2008. Calibration of a bio-optical model in the North River, NC: A tool to evaluate water quality impact on seagrasses. Estuaries and Coasts 31: 177-191, DOI 10.2007/s12237-007-9023-6.
[5] Zimmerman, R. C., V. J. Hill, and C. L. Gallegos. 2015. Predicting effects of ocean warming, acidification and water quality on Chesapeake Region eelgrass. Limnol. Oceanogr. 60: 1781-1804, doi: 10.1002/lno.10139.
[6] Gallegos, C. L., W. J. Kenworthy, P. D. Biber, and B. S. Wolfe. 2009. Underwater spectral energy distribution and seagrass depth limits along an optical water quality gradient. p. 359-368. In M. Lang and I. MacIntyre [eds.], Smithsonian Contribution to the Marine Sciences, No. 38. Smithsonian Institution Scholarly Press.
LONG-TERM CHANGES IN CHESAPEAKE BAY WATER CLARITY
While interacting with the Chesapeake Bay Modeling Subcommittee, I learned that my model for the diffuse attenuation coefficient was being evaluated using Secchi depth (ZSD) data. Knowing that the relationship between the Secchi depth and attenuation coefficient can be quite variable, this realization motivated me to analyze the long-term data on ZSD and the diffuse attenuation coefficient for photosynthetically active radiation (Kd(PAR)) for Chesapeake Bay. The relationship between the Secchi depth and Kd(PAR), and in particular the product of the two, ZSD·Kd(PAR), is governed primarily by the ratio of light scattering to absorption. We analyzed measurements of ZSD and Kd(PAR) at main stem stations in Chesapeake Bay and found that the ZSD·Kd(PAR) product declined at rates varying from 0.020 to 0.033 yr‑1 over the 17 to 25 years of measurements available at the time. This result implied that there had been a previously undiscovered long-term increase in the light scattering-to-absorption ratio. Bio-optical modeling in conjunction with remote sensing measurements implicated an increase in the relative proportion of organic detritus with high mass-specific scattering and low backscattering ratio. The paper highlights the lack of data to evaluate this conjecture.
Gallegos, C. L., P. J. Werdell, and C. R. McClain. 2011. Long-term changes in light scattering in Chesapeake Bay inferred from Secchi depth, light attenuation, and remote sensing measurements. J. Geophys. Res. 116: C00H08, doi:10.1029/JC2011007160, 19 pp.