| Abstract | This study was undertaken to determine the current quantity of phosphorus in the
sediments of St. Albans Bay, Vermont, USA, and to address the role of seasonal
variations related to changing redox conditions and mineralization on phosphorus cycling
and mobility in those sediments.
43 separate cores were collected in the first week of August, 2004 with the assistance of
the Vermont Department of Environmental Conservation to assess the overall
concentration and spatial distribution of phosphorus, iron, and manganese in the
sediments of St. Albans Bay. These cores were sectioned into 0-1, 1-2, 2-3, 3-4, 4-5, 5-8,
and 8-12 cm aliquots and their porosity, organic content, soluble reactive phosphorus
(NH4Cl extracted), mineralizable phosphorus (NaOH extracted), residual inorganic
phosphorus (HCl extracted), acid-extractable iron and manganese (HCl extracted),
amorphous Fe and manganese (ascorbic acid extracted), reactive phosphorus (ascorbic
acid extracted), and total iron, manganese, and phosphorus (aqua regia extracted)
determined. Statistical analysis of the data indicates that the mobility of phosphorus is
tied to the mobility of iron and manganese, especially in the top 2 cm of sediment. The
total amount of phosphorus tied up in the bay is significant; in the top 10 cm there are
approximately 1200 tons of phosphorus associated with amorphous iron and manganese
oxyhydroxide minerals, and over 4000 tons of total phosphorus. An original purpose of
the study was to compare phosphorus concentrations in cores collected during 2004 with
cores sampled by previous studies in 1982 and 1992. However, comparison to previous
studies in a direct, quantitative fashion was determined to be inappropriate due to
problems with how sampling results would be influenced by redox front positions and to
potential mobility of phosphorus collected by the various studies at different points in
time during the summer season, potential problems with analytical procedures in older
reports not comparable to newer, more accurate methods employed here, spatial
heterogeneities in the system, and indeterminate sedimentation rates for different parts of
the bay.
A total of 10 separate sampling excursions were undertaken between late May and early
October of 2004 in order to gather data to determine the seasonal changes in redox
chemistry within the top few centimeters of sediments using in situ electrochemical
methods to investigate porewater iron, manganese, sulfur, and oxygen chemistry.
Sediment core samples were additionally sectioned and analyzed to determine the
distribution of iron, manganese and phosphorus for a comparative study looking at the
relative mobility of these elements over this time period. It is well known that Fe and Mn
oxyhydroxide minerals strongly sorb phosphorus to their surfaces (Shenker et al., 2005;
van der Zee et al., 2003; Roden and Edmonds, 1997).
When these minerals are reduced due to conditions in the sediments becoming more
anoxic, the sorbed phosphorus can be released into adjacent porewaters where it may
diffuse and potentially reach the overlying water column - where it would serve as a
nutrient source, potentially driving algal activity. Electrochemical results show that the
comparatively colder and windier conditions of summer 2004 kept iron oxyhydroxide
minerals in surficial sediments of the inner bay from being reduced, but that manganese
oxyhydroxide minerals were completely reduced up to the sediment-water interface and
soluble Mn was observed in the water column. This observation is coupled with
statistical analysis of the sediment core chemistry from our seasonal site which suggests
that phosphorus mobility is strongly correlated with changes in iron and manganese
mineralization. While manganese minerals can release significant Mn into the porewater
and overlying water columns, the continued presence of oxidized iron oxyhydroxides
through the summer should have effectively contained any significant phosphorus
released from manganese oxyhydroxide minerals. However, in summers where
conditions may select for more reducing conditions to drive iron reduction (generally
higher temperatures, less turbulence from wind shear, and greater photosynthetic
activity), or in parts of the bay where conditions select for greater anoxia, the redox front
may move well into the water column, transform a more significant proportion of
oxidized iron and manganese oxyhydroxide minerals, and release substantial phosphorus
into the overlying water column.
In summary, the results of this study indicate that there remains a substantial reservoir of
phosphorus in the sediments of St. Albans Bay which is mobile within the sediment
column due to changing redox front positions and associated changes in iron and
manganese mineralization. This sediment reservoir has the potential to contribute
phosphorus to the water in the bay for a long period of time into the future; flux of
sediment into the overlying water column will be at least partly based on highly reducing
events which may vary considerably in space and time.
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