Spatial distribution of microplastics in a coastal upwelling region: Offshore dispersal from urban sources in the Humboldt Current System

In coastal waters, higher concentrations of microplastics (MPs) are generally related to densely populated and industrialized areas, but intense upwelling and offshore transport in the Eastern Boundary Upwelling Systems (EBUS) may influence this pattern. The Humboldt Current System (HCS) along the coast of northern-central Chile represents a perfect model to test whether the abundance of MP at the sea surface decreases with distance from land-based sources, e.g., river mouths, harbors, and submarine wastewater outfalls. The sea surface was sampled with a manta trawl to examine the abundance, composition, and distribution of floating MPs, and Generalized Additive Mixed Models (GAMMs) were performed to examine the relationship between MP abundance (particles km-2) and the distance to putative sources. MPs were found in all 57 net tows, with an average of ⁓120,000 MP km-2 and maximum values of ⁓1,500,000 MP km-2. The composition of MPs was dominated by fragments (>50% of the total count) and over 80% of all MPs were ≥1 mm. The combined effect of the various sources, spatially concentrated in urban areas, makes it difficult to distinguish their relative contributions, but the MP composition suggested that rivers are more important sources, followed by submarine wastewater outfalls and then harbors. A significant and steep negative relationship with the "distance to source" explained 15.2% of the variance of "MP abundance", suggesting rapid offshore displacement within the HCS. This is the first study to report this pattern along the edges of the South Pacific Subtropical Gyre (SPSG), revealing that continuous offshore transport of microplastic from land-based sources is occurring over large scales and contributing to the accumulation of microplastics in the center of the SPSG. However, the findings additionally suggested that processes at meso- and submeso-spatial scales (driven by geographic and seasonal variables) are disrupting the general pattern.

A perfect storm: An anomalous offshore phytoplankton bloom event in the NE Atlantic (March 2009)

While primary productivity is more stable in oceanic regions, it may vary to a great extent with the proximity to coasts, where mesoscale processes may intertwine and shape phytoplankton community composition and biomass. Sometimes, this may lead to the development of anomalous phytoplankton blooms (i.e., episodic blooms that exceed several times the average phytoplankton biomass). A massive bloom observed off the Western Iberian Coast (SW Europe) during March 2009 prompted a full investigation on its spatial and temporal extent, its causes, and its potential impact on the ecosystem. Results revealed that the March 2009 bloom was both novel in terms of biomass in a regional context and one of the largest anomalous blooms until now described in terms of relative magnitude. Its causes were due to a concurrence of long-term (deep winter MLD) and short-term factors (coastal upwelling, sudden changes in the water column, consistent offshore water transport). Its impact on the regional ecosystem is difficult to gauge, although the high concentrations of particulate organic carbon at surface during the bloom period suggests that it may have had a significant local impact. Since climate change is expected to increase the frequency and intensity of extreme weather events, it is possible that anomalous blooms will also become more frequent, expanding their role in shaping carbon export and food webs. These results are crucial for the monitoring of the Western Iberian Coast and are applicable to other complex coastal upwelling regions where phytoplankton biomass and variability have a crucial link to fisheries.

Using medically-derived iodine-131 to track sewage effluent in the Laurentian Great Lakes

Tracking sewage wastewater in a large lake is difficult. Concentrations of pharmaceuticals that can be used as indicator compounds are quickly diluted and not easy to measure. In this study, we examined the potential of using medically-derived iodine-131 (¹³¹I, t_½ = 8.02 d) as a tracer for Milwaukee sewage effluent in Lake Michigan. ¹³¹I activities in sewage effluent from two Milwaukee wastewater treatment plants (WWTPs) were measured in conjunction with ¹³¹I activities in water, sediment and biota in the Milwaukee Outer Harbor and Lake Michigan. ¹³¹I discharge rates from both WWTPS ranged from 34 ± 15 to 1807 ± 24 MBq d^-1, with average and median ¹³¹I discharges of 278 and 129 MBq d^-1. A budget of ¹³¹I in the Milwaukee Outer Harbor - based on measured sediment and water column inventories - showed that ∼11% of the ¹³¹I discharged to the harbor was scavenged to bottom sediments, ∼19% decayed in the harbor water column, and ∼70% was flushed out of the harbor to Lake Michigan. From this budget, we derived a harbor flushing rate of 3.1 days. In Lake Michigan, ¹³¹I activity was found in Cladophora algae (undetected to 91 ± 2 Bq kg^-1) along ∼40 km of shoreline. Benthic trawl samples showed ¹³¹I activity up to 8 km from shore. Calculated ¹³¹I length scales were 30 km alongshore and 3.4 km offshore and corresponded to sewage effluent dispersion rates of ∼2.6 km d^-1 and ∼0.3 km d^-1 in along- and offshore directions. Using ¹³¹I as a tracer of sewage effluent from other coastal municipalities to the Laurentian Great Lakes appears feasible, particularly for larger (>10⁵) population centers.