Environmental microplastic concentrations and uptake in selected rocky shore invertebrate filter-feeders, grazers and predators in False Bay, South Africa

Abstract

Plastic pollution in an ever-increasing threat globally and poor waste management in South Africa has caused an increase of plastic pollution in the environment. Microplastics (MPs) are plastic particles smaller than 5 mm in size, and the little to no research has been done on MP pollution within the marine coastal environment and rocky shores in South Africa. Sampling was done in February 2020 at a rocky shore within Simon’s Town Marina, Cape Town. Water (n = 5), sediment (n = 5) and biota (n ≤ 30) samples were collected. Biota samples were split between various feeding strategies, with filter feeders, grazers and scavengers/predators sampled to determine whether feeding strategy affects the potential uptake of MP particles. Water, sediment, and biota samples were digested in 10 % potassium hydroxide (KOH) and then extracted over a 20 μm mesh. MPs were extracted from sediment using the density separation method. Once isolated, MP particles were classified according to type, size and colour, and further classification was done using an FTIR-ATR according to polymer type. The results show that a higher concentration was found in sediment (38 ± 2 MP/kg) compared water (0.37 ± 0.056 MP/L) as the area has low water energy, allowing MP particles to settle within the sediment. For biota samples, filter feeders had the lowest average MP particle concentrations (0.28 ± 0.04 MP/g) but displayed the highest variation of MP particle colours due to the non-selective feeding strategy, where other feeding strategies ingested mostly black/grey particles. Grazers had the highest MP particle concentration (1.45 ± 0.27 MP/g), and this was due to grazers feeding on contaminated algae closer to the benthic regions where MP particles have settled in the sediment. Scavengers/predators showed lower concentrations of MP particles (1.08 ± 0.21 MP/g) but are more dependent on the trophic transfer of MP particles through detritus and prey instead of feeding on MP particles directly from the water column. Majority of the MP particles found in environmental and biota samples were between 100 μm and 500 μm in size. The most abundant MP polymer type was nylon (27.27 %), as well as and PET and natural MP particles such as cotton (18.18 %). Antifouling paint particles were also found, and this is due to the boat building and maintenance facility that is in close proximity to the sampling site. A risk assessment was also conducted and indicated that polymer type poses a greater risk of MP pollution than MP concentrations. An experimental study was also done, exposing organisms to MPs of known sizes. Not much is known about the ingestion rates, preferred size of MP ingestion and the relationship between the soft tissue weight (g) and microplastics per individual (MP/I) on microplastic ingestion by marine organisms in Cape Town. The blue mussel (Mytilus galloprovincialis) was sampled in December 2021 acclimatised to environmental conditions within a laboratory setting for 48 hours. Microplastic filaments (MPFs) were created and filtered using 10 μm-filtered reverse osmosis (RO) water through a vertical multi-tiered filtering system, creating MPFs of two different size classes, between 65 μm and 500 μm (small), and 500 μm and 1000 μm (large). These size classes were chosen to represent realistic sizes found in the environment according to the field study. Mussels were placed in glass beakers, in one litre of 20 μm-FSW with 50 MPFs per beaker. Half of the beakers were exposed mussels to smaller MPFs, and the other half exposed mussels to larger MPFs. At predetermined time intervals after initial exposure, mussels from the beakers for each MPF size class were removed and analysed for MPFs. The results showed that the highest concentration of MPFs in the soft tissue was observed at 3 hours after initial exposure (7.21 ± 1.69 MP/g) and after 1 hour after the initial exposure for MP/I 23.16 ± 4.16 MP/I). This could be due to plastics needing time to be dispersed throughout the water. The results also showed that mussels preferred MPFs of the larger size (500 μm – 1000 μm). The concentration of MPFs in the size between 500 μm and 1000 μm was also the highest at 180 minutes (8.91 MP/g) and again at 60 minutes for MP/I (27.33 MP/I). For the smaller MPFs (65 μm – 500 μm), the peak ingestion was seen at 180 minutes for the soft tissue weight and at 60 minutes MP/I, reinforcing that peak ingestion rates occur between 60 and 180 minutes after initial exposure. These studies are useful as it provides insight in the levels of microplastic pollution in a rocky shore in Cape Town, and also the potential sources of pollution, and also the preferred ingested MPF size in mussels in Cape Town, as well in when mussels are at the peak feeding rates after initial exposure to microplastic. The study provides reason to the importance of the combination of field and experimental studies in microplastics research and provides motivation or more studies to be undertaken.