Interannual variation in submerged aquatic vegetation and its relationship to water quality in subestuaries of Chesapeake Bay

Abstract

The abundance of submerged aquatic vegetation (SAV) fluctuates from year to year, and these fluctuations differ among the tributary embayments within estuaries. Quantifying the causes and spatial patterns of interannual variability in SAV abundance is critical to understanding the dynamics of SAV in complex estuaries. We applied empirical orthogonal function (EOF) analysis to quantify the temporal synchronicity among embayments within each of 3 salinity zones (oligohaline, mesohaline, and polyhaline) of Chesapeake Bay. EOF analysis extracts shared modes of variation among the embayments in each zone. The amount of synchronicity differed among zones, and embayments were most synchronous within the polyhaline zone. The extracted modes of variation showed that fluctuations in SAV abundance are not synchronized among the salinity zones. We detrended the modes of common variation in SAV abundance for each salinity zone and then used cross-correlation analysis to compare the detrended modes to detrended water quality variables (dissolved organic carbon, suspended solids, chlorophyll a, Secchi depth, river nitrogen load, temperature, and salinity). The water quality variables that were significantly related to SAV abundance differed among salinity zones. Chlorophyll a levels in months when seed germination and shoot production rates are high were important negative predictors in the oligohaline and polyhaline zones, possibly because young SAV plants are especially sensitive to even short-term disturbances. As the SAV communities in the 3 salinity zones have different yearto-year dynamics and different responses to stressors, conservation, restoration, and management plans should be tailored to each community.

Land Use and Salinity Drive Changes in SAV Abundance and Community Composition

“…Similarly, incorporating species and community information into SAV models may clarify confusing relationships reported between terrestrial stressors and total SAVabundance (Kemp et al 2004;Patrick and Weller 2015). Land use is hypothesized to impact SAV when it promotes suspended sediments and eutrophication that reduce light availability or when shoreline land use reduces potential habitat by changing nearshore wave energy and eroding shallow water habitat (Koch 2002;Strayer and Findlay 2010;Gittman et al 2015).…”

“…Land use is hypothesized to impact SAV when it promotes suspended sediments and eutrophication that reduce light availability or when shoreline land use reduces potential habitat by changing nearshore wave energy and eroding shallow water habitat (Koch 2002;Strayer and Findlay 2010;Gittman et al 2015). Species-specific differences in habitat needs may affect the strength of relationships between anthropogenic stressors and SAV abundance (Batiuk 2000;Kemp et al 2004;Patrick and Weller 2015). Stressors can also eliminate sensitive species so that species composition shifts to tolerant or invasive species even if coverage does not change.…”

“…Bay-wide SAV abundance has been mapped annually since 1984, and prior research informs our understanding the system (Orth et al 2010;Ruhl and Rybicki 2010;Williams et al 2010;Gurbisz and Kemp 2014;Patrick et al 2014). The many tributary subestuaries to the bay provide replicate study units with strong differences in human land use (agricultural or urban land cover or shoreline armoring), and different regions have different temporal trends and interannual fluctuations (Orth et al 2010;Patrick and Weller 2015). The rich spatiotemporal data and the wide range of stressor conditions facilitate testing how stressors interact with communities through time.…”

Local and regional disturbances associated with the invasion of Chesapeake Bay marshes by the common reed Phragmites australis

“…Thus, the subestuaries provide a population of replicate study units well suited for exploring the interacting effects of land use and salinity on estuaries. That replication has been exploited in recent statistical analyses relating watershed and subestuary characteristics to ecological responses in the subestuaries (King et al 2004(King et al , 2007Li et al 2007;Patrick et al 2014Patrick et al , 2016Patrick and Weller 2015).…”

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“…Alongside increasing industrialization of the region in the 1960s, there emerged interest in 71 the impact of human activities on eelgrass in Chesapeake Bay: specifically, nutrient runoff from 72 agriculture, and the consequent eutrophication of nearshore waters (Orth & Moore, 1984;Kemp et 73 al., 2005). Several recent correlative analyses have proposed that declining water quality and 74 subsequent changes in light availability may be the preeminent agent preventing recovery of 75 eelgrass in Chesapeake Bay after Agnes (Orth et al, 2010;Patrick & Weller, 2015). At the same time, 76 parallel investigations conducted in only a single sub-estuary have uncovered a potential role for 77 rising temperatures alongside reduced visibility in driving a recent decade-long decline of eelgrass 78 (Moore & Jarvis, 2008; Moore et al, 2014).…”