Scientists and Regulators Grapple with Scarborough’s Water Quality Puzzle

Two scientific reports attempting to untangle the complex web of factors affecting bathing water quality in Scarborough’s North and South Bays have come to differing opinions on the impact of Seagulls.

Water quality experts have presented findings that, while agreeing on the major role played by local watercourses, remain sharply divided over the contribution of the town’s famous seabird population.

The investigations were launched to address a stark geographical divide: while Scarborough North has a history of "Excellent" or "Good" water, it has recently slipped to "Sufficient," whereas Scarborough South has struggled with a long history of "Poor" classifications. Two distinct studies were commissioned to find the "smoking gun": one led by Professor Darren Grocke of Durham University, using nitrogen isotope analysis of seaweed, and another by the Environment Agency (EA), which utilised DNA-based bacterial profiling.

The Source of the Problem

Both reports identify Scalby Beck, which flows around the northern edge of Scarborough, as a primary conduit for pollution. Professor Grocke’s research, which involved collecting over 3,400 seaweed specimens between 2023 and 2025, indicates that nitrogen from the Beck travels southward due to ocean currents, ultimately becoming trapped in the curved geography of South Bay.

The Professor’s analysis suggests the primary culprits are a mix of animal manure and human sewage. Regarding his findings on the specific type of nitrogen entering the water, Professor Grocke said:

"It all points to animal or human sewage as the source of nitrogen that's coming around the coastline.

There is no input of sea bird guano. We're not seeing that.

We're not seeing sort of any indication of fertilizers or agricultural fertilizers coming in.

We have also been doing e.coli nitrate analyses all this through the coast when we've been collecting and they all also indicated that there was human sewage coming in as well as animal sewage".

Professor Grocke explained that the presence of lush, green seaweed at Scalby Beck—often mistaken by the public as a sign of a healthy ecosystem—is actually a red flag for high nitrogen levels. He stated:

"Even just visiting the sites over the year, we could see dramatic changes. And I know it's a little bit non-scientific, but through all the works that I've been doing around many other coasts of the UK. We started going to sites and we could see lots of green seaweed. And we thought, ah, this is really clean. This is really healthy. You know, these look healthy plants.

Whereas in fact, that actually is reverse psychology. It was actually polluted because they were influenced by same as your plants. If you give it lots of nitrogen, it will grow bigger and look healthier.

And you can see that nearly all the time around Scalby Beck and the outlook from Scabby Beck. It sometimes it's completely green with sea ulva".

The Environment Agency’s Perspective

While the EA also identified human sewage as a consistent factor, their investigation painted a broader picture of the biological markers present in the water. Martin Christmas, the EA’s Area Environment Manager, confirmed that while human bacteria are a constant in both bays, the South Bay is subject to a wider variety of inputs.

Highlighting the specific bacterial markers found during their monitoring, Mr Christmas said:

"So for South Bay, we're picking up human and seabirds sources bacteria constantly present. Occasionally, we're finding dogs and very occasionally cows and sheep in South Bay. Donkeys seem to be exonerated from this situation.

And for North Bay, we're picking up markers that show human and seabird sources of bacteria consistently present. And the seabird markers are at much lower concentrations compared to those found in Scarborough South.

The signal strength of the diagnostic tool does suggest that the seabirds in South Bay are more of an influence than seabirds in North Bay".

The Seagull Discrepancy

The most striking point of contention between the two reports is the impact of seagulls. While the EA maintains that seabird DNA is "constantly present," Professor Grocke’s isotope study found no evidence that bird waste was contributing significantly to the nitrogen load in the seaweed.

Mr Christmas addressed this scientific disagreement directly, suggesting that the different methodologies used by each team might explain the conflicting results. He noted:

"Professor Grocke is using a different technique and this is why the situation is complicated.

He would say it's not seabird guano using the techniques that he's looking at. What I'm going to tell you is that seabird DNA markers are always present at South Bay and North Bay.

So you have to take those two pieces of evidence together. You can't just say because one piece of science has said one thing that you disregard another. It's just a fact from using different monitoring techniques and tools".

Industrial and Urban Inputs

Beyond biological waste, the reports touched on industrial impacts. Treated effluent from both the Scarborough Wastewater Treatment Works and McCain Foods has been detected in both bays at various times, confirming a complex "north-to-south" and "south-to-north" link between the two locations. However, McCain Foods has recently invested £23 million in a state-of-the-art wastewater facility that removes over 97% of organic material, ensuring their discharge now meets high environmental standards.

In the North Bay, a specific link was also indicated between the waters of Peasholm Lake and the beach. Furthermore, Scalby Beck remains a major focus due to seven Yorkshire Water combined sewer overflows (CSOs) that discharge there during heavy rain.

Future Actions

The EA plans to install five continuous water quality monitors in Scalby, Cow Wath, and Peasholm Becks in 2026, alongside targeted inspections of farms and private discharges from hotels and caravan sites.

For their part, Yorkshire Water has committed to an accelerated investment programme, including a new reduction tank near Jackson’s Bay, to limit storm overflow spills to just two per season by 2030.