Many filter-feeding invertebrates consume microplastics (MP) under laboratory conditions, but little is known about newly settled, field-collected juveniles. To address this information gap, we collected 3439 juvenile invertebrates in the Indian River Lagoon (IRL), FL, USA. Previous studies suggest that the IRL is a MP hotspot. A total of 70% of IRL adult oysters (Crassostrea virginica) contained MP (mean: 2.3 MP/individual), and MP number and MP length were positively correlated with animal size. We predicted that juvenile C. virginica and other sessile invertebrates would contain MP with a positive correlation to animal size. Five species were examined; 51% were C. virginica (mean shell length ± SD: 6.3 ± 4.7 mm). Overall, 117 (3.4%) animals contained potential MP (fibers: 90.7%). Of these particles that matched FTIR databases with a score of 70% or greater, 51% were plastic and 49% were anthropogenically modified particles. No correlations to animal size were found for particle presence (logistic regressions: p ≥ 0.20 for all species) or particle length (linear regressions: p ≥ 0.23 for all species). Thus, even though found in a MP hotspot, our extrapolated results suggest few juveniles (<1%) contained MP. This information is important for understanding the relationship between MP and the life histories of filter-feeding animals, especially for species considered biological indicators of MP.

Introduction: Seagrass restoration efforts have been motivated by global decreases in coverage associated with anthropogenic influences on the environment. In the Indian River Lagoon (IRL), FL, USA, seagrass declined by 75% between 2011 and 2021. During this time, abundance of the native macroalga Caulerpa prolifera increased substantially in this system. As restoration efforts began in the IRL, it became apparent that varying abundance of C. prolifera could impact the recovery of seagrass. Halodule wrightii is the most abundant seagrass in the IRL and often targeted for restoration use. Methods: Experiments were done using outdoor, recirculating-water mesocosms to test the effects of C. prolifera abundance on planted H. wrightii . Results and Discussion: Results showed a competitive effect on outcomes of H. wrightii transplants when C. prolifera occurrence is 90%. Within the context of restoration, this indicates that newly transplanted fragments of H. wrightii could struggle to establish themselves due to resource deficiencies in areas with high coverage (90% occurrence) of C. prolifera . Mixed results with medium coverage of C. prolifera (65% occurrence) and no effect in areas with low coverage (25% occurrence) indicate a potential transition range above 65% occurrence and below 90% occurrence of C. prolifera where consistent effects of competition begin to appear on H. wrightii growth. Therefore, the success of transplanted H. wrightii fragments into low-to-medium C. prolifera abundance is not likely to be limited by competition with this native macroalga.