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SCOPE OF SERVICES
Background: Past studies of Lake Pontchartrain indicated a stressed benthic invertebrate
community. Causes of stress were believed to be shell dredging and toxic compounds in the
sediment. Recent studies indicate that recovery from shell dredging has occurred and that
sediment contamination is not a major problem, but that saltwater intrusion and episodic
hypoxia are serious problems. Unnaturally high salinity bottom water has entered Lake
Pontchartrain since the Mississippi River Gulf Outlet was completed in 1963. This saline
water produces stratification and bottom hypoxia. Low benthic invertebrate species diversity
and the absence of large (> 20 mm) Rangia cuneata clams from a 100 mi 2 north of the IHNC
documented in previous studies demonstrated a major impact. During the drought associated
with the La Niña climate pattern from 1998 to 2002, benthic invertebrate species diversity
and Rangia clam density decreased lake-wide from 374 to 4 clams/m 2 . This decrease in
species diversity and in Rangia abundance throughout the lake may have been due to
episodes of lake-wide low dissolved oxygen. Salinity has since returned to normal, but
Rangia clams have not recovered. Additional studies are needed during 2004 and 2005 to
obtain information needed to determine causal factors. These studies would provide
information on the effects of salinity shifts on the overall benthic community; possible
occurrence and effects of lake-wide hypoxia resulting from salinity stratification; the rate of
benthic invertebrate recovery, especially Rangia clams; and Rangia growth rates.
Goals: The long-term goal is to provide the knowledge needed to restore the benthic
community. Cases of decline and recovery need to be determined and natural variation
separated from human-induced changes. The working hypothesis is that saltwater intrusion is
the primary cause of stress, but additional data on the extent and degree of stress and benefits
of restoration are needed. Information on the effects of Mississippi River diversions is also
needed. The primary goal for 2004-2005 studies is to determine the cause of Rangia decline
and the rate of recovery.
Objectives: (1) Determine the status of the bottom biota of Lake Pontchartrain by
conducting lake-wide surveys of the distribution and abundance of benthic invertebrates
during 2004 and 2005. (2) Relate 2004-2005 data to historic data to determine status and
trends. This analysis will focus on the effects of hypoxia from salinity stratification, but will
also include possible changes due to spillway openings/leakage, shell dredging, toxic
chemicals, eutrophication, and shifts in overall ecosystem structure. (3) Determine changes
in the population dynamics of Rangia clams due to drought driven salinity changes and
recovery after a return to normal rainfall. (4) Based on benthic-invertebrate indicator species,
evaluate the current area affected by hypoxia from salinity stratification in 2004 and 2005
(100 mi 2 in 1999).
Project Methodology: Materials and methods employed in past UNO studies of Lake
Pontchartrain benthic invertebrates will be used in this study. Sites on established transects
will be sampled (three replicates) with a petite Ponar grab, sieved through a 0.6 mm screen
bucket, preserved in formalin with rose Bengal stain in the field, and sorted and identified in
the laboratory. Statistical analyses of data will include ANOVA and analyses in PRIMER
(Plymouth Routines in Multivariate Ecological Research). All comparisons of data sets,
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