Increases in Cyanobacteria during Summer in Mesohaline Chesapeake Bay, 1985-2006Ann Marie Hartsig, R.V. Lacouture, A.L. Imirie1 1Morgan State University Estuarine Research Center, St. Leonard, MD USA Results Abstract Phytoplankton species composition has been taxonomically enumerated in the Bay and its tributaries since August, 1984 as part of the Chesapeake Bay Water Quality Monitoring Program. A significant increase in cyanobacteria biomass has been observed in the mesohaline portion of the estuary during the past decade in the summer months. The increase which has been documented over the long-term has been within the nano-micro component of the phytoplankton and is almost exclusively composed of several filamentous taxa. These filamentous cyanobacteria often exceed 300 µgC/L in the mid and lower Maryland mainstem Bay and 500 µgC/L in the lower Potomac River. During peak concentrations, biomass of these filaments can attain over 1000 µgC/L. Autotrophic picoplankton are the other primary portion of the cyanobacteria population during the summer and have been quantified since 2002. The biomass of this part of the cyanobacteria assemblage usually ranges from 50-200 µgC/L. Peak biomass of picoplankton can exceed 500 µgC/L. The total cyanobacterial carbon comprises 20-30% of total phytoplankton carbon in the lower reaches of the MD tributaries and 40-50% in the mainstem Bay. In several analyses, this increase in cyanobacteria does not appear to have a detrimental impact on copepod feeding. SUMMER NANO-MICRO CYANOBACTERIA CARBON MAINSTEM CHESAPEAKE BAY 1985-2006 • The nano-micro fraction of thecyanobacteria biomass often exceeds 300 µgC/L in the mid and lower MD mainstem Bay and in the lower Potomac River concentrations often exceed 600 µgC/L • The nano-micro size fraction of the cyanobacteria consists of several colonial and filamentous forms • A few thin, filamentous forms (tentatively named Pseudanabaena, • Planktolyngbya) dominated mesohaline regions beginning in 1994 Cyanophytes Found in the Mesohaline Regions of Chesapeake Bay and Tributaries Oscillatoria sp., Merismopedia sp., cf. Planktolyngbya sp., cf. Pseudanabaena limnetica Introduction The Chesapeake Bay Water Quality Monitoring Program was initiated in 1984 as a vital component of the management of the ecosystem. The focus of the monitoring program is to evaluate a suite of parameters important to the health of the ecosystem. These parameters include nutrient loads, water quality, plankton biomass and community structure, and SAV habitat requirements and distribution. Since 1984, the Morgan State University Estuarine Research Center (formerly the Academy of Natural Sciences Estuarine Research Center) has characterized quantitative and qualitative aspects of phytoplankton communities in different salinity regimes within the estuary. • Concentrations of cf.Pseudanabaena limnetica increased substantially in 1994 and have remained relatively high since that time • Peak concentrations of cf.Pseudanabaena limnetica generally attain 300-400 µgC/L Autotrophic Picoplankton Methods Over the past 23 years, phytoplankton species composition, biomass and production have been measured in the surface mixed layer of the mesohaline Chesapeake Bay and the mesohaline tributaries stations (CB3.3C, CB4.3C and CB5.2, LE2.2-Potomac River, ET5.2-Choptank River, XDE5339-Patuxent River, WT5.1-Patapsco River) bi-weekly from April-September, and monthly during the remainder of the year. Aliquots of Lugols’ preserved samples were settled using the Utermohl gravitational settling technique. The nano-micro size phytoplankton were identified and enumerated at 500X, 312X and 100X magnificationusing a Leitz Diavert inverted microscope. In 2002, autotrophic picoplankton sampling began for summer months in the mesohaline portion of the Bay as well. Samples fixed in glutaraldehyde were filtered onto a slide and examined using a Leitz Laborlux compound microscope outfitted for epifluorescence. Cells are enumerated at 1250X magnification. Densities (cells/L) were converted to biomass ( µgCarbon/Liter) based upon carbon estimates of individual taxa which were calculated from biovolume and specific equations for different phylogenetic groups. • The biomass of the pico size fraction of the mesohaline cyanobacteria populations is generally 50-200 µgC/L during the summer in the MD portion of the estuary and highest at the stations in the higher salinity areas Phototrophic picoplankton cells autofluorescing under an epifluorescent microscope
Discussion • An increase in cyanobacteria in the mesohaline regions of Chesapeake Bay and its tributaries has been documented • Possible factors driving the increase of cyanobacteria were analyzed: DIN, PO4, N:P, Secchi and Salinity for station LE2.2 • Regression analysis results indicate weak relationships between these parameters and cyanobacteria biomass (see table below). Salinity at LE2.2 showed the strongest relationship (r2=0.38). Combined with the spatial pattern discerned in the variability in filamentous carbon at the sampling sites, there may be a preference for higher salinities by these filamentous cyanobacteria -- (cf. Pseudanabaena limnetica, cf. Planktolyngbya sp.) • When the two cyanobacteria fractions are combined, the total cyanobacteria biomass is generally 100-400 µgC/L but peaks at certain sites exceed 600 µgC/L • Station CB5.2 in the lower MD Bay and LE2.2 in the lower Potomac River are characterized by the largest totals for cyanobacteria biomass • Total cyanobacteria biomass concentrations exceed 400 µgC/L in 36% of the sampling records at CB5.2 and 52% of the records for LE2.2 • Peak cyanobacteria carbon totals exceeded 1000 µgC/L on one occasion at CB5.2, during two occasions at LE2.2 and once at WT5.1 in the Patapsco River • cf. Pseudanabaena limnetica dominated the total biomass in all instances Potential Trophic Impact • It appears that the overall quantity of phytoplankton as food is more than sufficient to support a copepod population. The total useable phytoplankton carbon (all nano-micro carbon except cyanobacteria) exceeds a threshold of 150 µgC/L which is the minimum quantity to support a population of copepods. • An examinations of the proportion of filamentous cyanobacteria encountered by a copepod, like Acartia is minimal, rarely exceeding 10% of the total cells in the water column.Thus, the energetics of a copepod’s search for food is likely not severely impacted by the filaments. • The percentage of total phytoplankton carbon comprised of cyanobacteria increased from upbay (CB3.3C) to midbay (CB4.3C) to downbay (CB5.2) • In the MD tribs the percentage of total phytoplankton biomass made up of cyanobacteria during the summers 2002-2006 was generally highest at LE2.2 (lower Potomac River) • There was at least one sampling record for each station in which the cyanobacteria biomass made up at least 50% of the total phytoplankton carbon References Anagnostidis, K. and Komàrek, J. 1988. Modern approach to the classification system of cyanophytes. 3-Oscillatoriales. Arch. Hydrobiol. Suppl. 80, 1-4. Algological Studies 50-53 Druet, F. 1968. Revision Of the Classification of The Oscillatoriaceae. Monograph 15. The Academy of Natural Sciences of Philadelphia pp. 349 Geitler, L. 1932. Cyanophyceae. Koeltz Scientific Books for Edition. Federal Republic of Germany. pp 1195 Humm, H. J. and Wicks, S. R. 1980. Introduction and Guide To The Marine Bluegreen Algae. A Wiley-Interscience Publication. John Wiley and Sons. New York. pp 193. Klein-Breteler, W.C.M., H.G. Fransz, and S.R. Gonzalez. 1982. Growth and development of four calanoid copepod species under experimental and natural conditions. Netherlands Journal of Sea Research 16: 195-207. Komárek, J. and Anagnostidis, K. 2005. Cyanoprokaryota 2. Teil: Oscillatoriales 19/2, Germany pp. 759. Komárek J. & Hauer T. (2004): CyanoDB.cz - On-line database of cyanobacterial genera. - http://www.cyanodb.cz Prescott, G. W. 1982. Algae of the Western Great Lakes Area. Otto Koeltz Science Publications, West Germany. pp 969. Percentage of total phytoplankton carbon comprised by nano, micro and pico cyanobacteria 2002-2006 • The percentage of total phytoplankton carbon comprised of cyanobacteria ranged from 25-50% in the mainstem Bay and 20-40% in tributaries • Highest percentages in the Bay occurred in September, 2002 with 90% at CB3.3C, 73% at CB4.3C in July, 2004 and 80% at CB5.2 during August, 2005 • Highest percentages in the tributaries occurred at LE2.2 during August, 2004 with 76% and September, 2005 at 76%; also during July, 2004 with 84% of the total phytoplankton carbon comprised of cyanobacteria Acknowledgements The authors gratefully acknowledge the Maryland Department of Natural Resources and the Chesapeake Bay Program for their continued funding of the phytoplankton monitoring portion of the Chesapeake Bay Water Quality Monitoring Program, Stella Sellner for the enumeration of the picoplankton samples, Dr. Guy Hallförs of the Finnish Institute of Marine Science and Walter Bulter from MD DNR for their expertise and assistance in the taxonomic identification of the filamentous cyanobacteria.