Seed addition facilitates eelgrass recovery in a coastal bay system

Eleven years of eelgrass Zostera marina seed additions conducted in a coastal bay system where Z. marina had not been reported since 1933 have resulted in rapid Z. marina expansion beyond the initially seeded plots. From 1999 through 2010, 37.8 million viable seeds were added to 369 individual plots ranging in size from 0.01 to 2 ha totaling 125.2 ha in 4 coastal bays. Subsequent expansion from these initial plots to approximately 1700 ha of bay bottom populated with Z. marina through 2010 is attributable to seed export from the original plots and subsequent generations of seedlings originating from those exports. Estimates of annual patch vegetative expansion showed mean estimated diameter increasing at varying rates from 10 to 36 cm yr(-1), consistent with rhizome elongation rates reported for Z. marina. Water quality data collected over 7 yr by spatially intensive sampling, as well as fixed-location continuous monitoring, document conditions in all 4 bays that are adequate to support Z. marina growth. In particular, median chlorophyll levels for the entire sampling period were between 5 and 6 mu g l(-1) for each of the bays, and median turbidity levels, while exhibiting seasonal differences, were between 8 and 9 NTU. The recovery of Z. marina initiated in this coastal bay system may be unique in seagrass recovery studies because of how the recovery was initiated (seeds rather than adult plants), how rapidly it occurred (years rather than decades), and the explicit demonstration of how one meadow modulated water clarity and altered sediments as it developed and expanded. Our results offer a new perspective on the role seeds can play in recovery dynamics at large spatial scales.