Michael F. Lane

Seasonal and spatial patterns of distribution of subtidal benthic invertebrate communities in the Mondego River, Portugal : a poikilohaline estuary

The use of benthic assemblages to assess ecological quality of estuarine environments is a major tool for the implementation of the Water Framework Directive (2000/60/CE) for European aquatic ecosystems. Benthic communities show spatially heterogeneous distributions and experience seasonal variations due to both natural and anthropogenic stresses. The major goal of this study was to quantify the relationships between environmental gradients and the spatial and temporal patterns of the benthic communities along a Portuguese estuary. Seasonal and spatial variations relating macrobenthic communities and measures of water and sediment quality along the northern branch of the Mondego River estuary were examined at seven sampling stations from July 2000 to June 2001. Cluster analysis of biological data indicated three major groups of communities based on spatial distribution patterns: (1) a lower sector with stronger marine influence and dominated by Streblospio shrubsolii and Cerastoderma glaucum; (2) a middle sector with dominance of S. shrubsolii and Corophium multisetosum; (3) and an upper sector where C. multisetosum dominates a community characterized by a lower number of species. Canonical correspondence analyses of biological and environmental data determined a major salinity gradient influencing benthic communities. Seasonal changes of benthic communities were mainly determined by freshwater input and salinity changes that imposed a strong decrease in densities and number of species during winter, followed by a recovery during spring. Benthic ecological structure and contaminant levels indicated that the Mondego northern branch is moderately disturbed, although opportunistic species dominated the benthic community, suggesting that natural and anthopogenic sources of stress may be acting together. The Mondego River estuary, a poikilohaline-type estuary, characterized by strong seasonal changes in water flow and salinity, cannot be consistently stratified into salinity regions based upon the Venice classi- fication system. Biotic communities, exemplified here by the benthic communities, are seasonally displaced, compared to a homiohaline-type estuary where the Venice system can be applied without modification. Future identification of reference conditions and design of monitoring programs cannot be accomplished without understanding how interactions between biotic and physico-chemical dynamics differ between homiohaline and poikilohaline estuaries. Results obtained in this study could be used to assist future assessments in other Portuguese estuaries.

Assessment and significance of phytoplankton species composition within Chesapeake Bay and Virginia tributaries through a long-term monitoring program

Phytoplankton and water quality long term trends are presented from a 20-year monitoring program of Chesapeake Bay and several of its major tributaries. Increasing phytoplankton biomass and abundance are ongoing within this estuarine complex, with diatoms the dominant component, along with chlorophytes and cyanobacteria as sub-dominant contributors in the tidal freshwater and oligohaline regions. Diatoms, dinoflagellates, and cryptomonads are among the major flora downstream in the tributaries and within the Chesapeake Bay. Water quality conditions within the three tributaries have remained rather stable over this time period; while there are long term trends of reduced nutrients, increasing bottom oxygen, and decreasing water clarity for the lower Chesapeake Bay. Of note is an increasing trend of cyanobacteria biomass at 12 of the 13 stations monitored at tributary and Chesapeake Bay stations, plus the presence of 37 potentially harmful taxa reported for these waters. However, the overall status of the phytoplankton populations is presently favorable, in that it is mainly represented and dominated by taxa suitable as a major food and oxygen source within this ecosystem. Although potentially harmful taxa are present, they have not at this time exerted profound impact to the region, or replaced the diatom populations in overall dominance.

Taxonomic sufficiency as a useful tool for typology in a poikilohaline estuary

Taxonomic sufficiency has been used mainly to assess benthic condition, based on the assumption that taxa can be identified to a taxonomic level higher than the species level without losing the ability to detect changes related to pollution stress. Identifying taxa to a higher level reduces the expertise and time needed to identify organisms and consequently allows increased spatial and temporal replication. The usefulness of taxonomic sufficiency for typology (identification of water body types) was examined using the benthic communities of the Mondego River estuary (Portugal). Benthic samples were collected seasonally along the Northern branch of the Mondego River estuary from July 2000 to June 2001 and several environmental parameters were measured simultaneously. Cluster analysis of species data indicated three major ecological groups, mainly related to a saline gradient along the estuary. The same groups were found when taxa were aggregated to higher taxonomic levels (genus, family, order, class), except for the phylum level. The overall spatial pattern was driven by: (1) the dominance of bivalves and the occurrence of rare marine species in the Lower Estuary; (2) the dominance of polychaetes in the Middle Estuary; (3) and the dominance of arthropods in the Upper Estuary. The ability of different taxocenes to discriminate the three ecological groups was also examined. Mollusca and Bivalvia were the only taxocenes producing the same groupings, although other taxocenes (Annelida, Polychaeta, Spionidae, Arthropoda) showed a significant ability to discriminate between all three groups. Compared to using all taxa identified to the lowest possible taxonomic level, our results indicate that for typology (1) several higher taxonomic levels were sufficient (2) while few taxocenes alone were sufficient.

Long-term phytoplankton trends and related water quality trends in the lower Chesapeake Bay, Virginia, U.S.A.

Long-term trends (i.e., 1985 through 1999; 14 1/2 yrs) of the phytoplankton community in Chesapeake Bay indicated patterns of increasing phytoplankton abundance and biomass associated with mainly diatoms and chlorophytes, and to a lesser degree dinoflagellates. Decreasing trends in productivity rates above the pycnocline were present over a shorter time period (10 1/2 yrs.), with evidence for increasing nitrogen limitation is indicated. Reduced light availability is inferred due to decreasing trends of Secchi depths and increased suspended solids trends, which were associated with decreasing trends in productivity rates.