Head‐of‐tide bottleneck of particulate material transport from watersheds to estuaries

Abstract

We measured rates of sediment, C, N, and P accumulation at four floodplain sites spanning the nontidal through oligohaline Choptank and Pocomoke Rivers, Maryland, USA. Ceramic tiles were used to collect sediment for a year and sediment cores were collected to derive decadal sedimentation rates using 137Cs. The results showed highest rates of short‐ and long‐term sediment, C, N, and P accumulation occurred in tidal freshwater forests at the head of tide on the Choptank and the oligohaline marsh of the Pocomoke River, and lowest rates occurred in the downstream tidal freshwater forests in both rivers. Presumably, watershed material was mostly trapped at the head of tide, and estuarine material was trapped in oligohaline marshes. This hydrologic transport bottleneck at the head of tide stores most available watershed sediment, C, N, and P creating a sediment shadow in lower tidal freshwater forests potentially limiting their resilience to sea level rise.

The Role of the Upper Tidal Estuary in Wetland Blue Carbon Storage and Flux

“…Soil C in tidal wetlands continues to leach after even a hundred years of burial (Maher et al, ), such that disparities between Pb‐210 and C‐14 profiling can be expected. Also, comparable to our Upper sites are C B estimates of 49–82 g C · m −2 · year −1 from the Altamaha, Ogeechee, and Satilla rivers, Georgia, United States (Craft, ) and 105–182 g C · m −2 · year −1 from along the Choptank and Pocomoke rivers, Maryland, United States (Ensign et al, ); our range of values for C B was broader.…”

“…Large rates of sediment trapping by upstream nontidal floodplain wetlands near the head of tide (and farther downstream at times) create a sediment shadow that limits sediment availability to lower reaches of TFFW (Ensign et al, ; Noe et al, ). However, C sedimentation from allochthonous riverine and autochthonous wetland sources still represented an important flux of C on our sites, ranging from 86 to 410 g C · m −2 · year −1 (Table ).…”

“…For example, soil C burial rates from tidal freshwater forested wetlands (TFFW) were 22–90 g C · m −2 · year −1 (Craft, ), and soil C burial from tidal freshwater marshes (124 g C · m −2 · year −1 ) exceeded burial from both low‐salinity marshes (93 g C · m −2 · year −1 ) and saltmarshes (40 g C · m −2 · year −1 ) (Loomis & Craft, ). Soil C burial in tidal freshwater forested wetlands in Maryland, United States was even higher, ranging from 105 to 182 g C · m −2 · year −1 (Ensign et al, ). With annual mineral surface sediment accumulation of C alone ranging from 86 to 410 g C · m −2 · year −1 in some cases (Noe et al, ), tidal freshwater and oligohaline wetlands likely play a key role globally in coastal C dynamics.…”

Floodplain Trapping and Cycling Compared to Streambank Erosion of Sediment and Nutrients in an Agricultural Watershed

“…; Ensign et al. ), including some noteworthy agricultural floodplains (average = 0.42 cm/yr, Trimble ). Sedimentation rates and P sedimentation rates in Smith Creek were high and most comparable to rates often observed in the Coastal Plain lowlands, where highly connected floodplains trap large amounts of river loads (Noe and Hupp ).…”

“…; Ensign et al. ). However, N sedimentation rates were generally lower than those measured in five types of floodplain communities (agricultural grasslands, seminatural grasslands, reedbeds, woodlands, and ponds) in an area characterized primarily by fluvial deposits, cover sands, and late Holocene marine and perimarine deposits (Figure : Kluiving et al.…”